|HOUSE OF LORDS
OPINIONS OF THE LORDS OF APPEAL FOR JUDGMENT
IN THE CAUSE
Chartbrook Limited (Respondents) v Persimmon Homes Limited and others (Appellants) and another (Respondent)
 UKHL 38
5. On 16 October 2001 Chartbrook Ltd (“Chartbrook”) entered into an agreement with Persimmon Homes Ltd (“Persimmon”), a well-known house-builder, for the development of a site in Wandsworth which Chartbrook had recently acquired. The structure of the agreement was that Persimmon would obtain planning permission and then, pursuant to a licence from Chartbrook, enter into possession, construct a mixed residential and commercial development (commercial premises below, flats above, parking in the basement) and sell the properties on long leases. Chartbrook would grant the leases at the direction of Persimmon, which would receive the proceeds for its own account and pay Chartbrook an agreed price for the land. Planning permission was duly granted and the development was built, but there is a dispute over the price which became payable.
6. Schedule 6 contained the relevant provisions. The price was defined as the aggregate of the Total Land Value and the Balancing Payment. The Total Land Value was made up of three parts: Total Residential Land Value, Total Commercial Land Value and Total Residential Cark Parking Land Value. Total Residential Land Value was to be £76.34 per square foot multiplied by the area for which planning permission for flats was granted. Total Commercial Land Value was £38.80 per square foot multiplied by the area for which planning permission for shops and other commercial uses was granted. And Total Residential Cark Parking Land Value was £3,024 multiplied by the number of spaces for which planning permission was granted. The Schedule set out the dates upon which the Total Land Value was to be paid. In principle, payment would fall due as each flat, shop or parking space was sold. But there was also a backstop provision for payment of specified percentages of the Total Land Value (so far as not already paid) by dates commencing about two and a half years after the grant of planning permission and ending about two years later, by which time the whole sum was due, whether the properties had been sold or not.
7. The provisions about Total Land Value are all quite straightforward and only require the insertion of the appropriate figures from the planning permission (which are not in dispute) into the formulae provided. The other element in the price is the Balancing Payment. For reasons concerned with its drafting history which need not be explored, the Schedule defines the Balancing Payment as the Additional Residential Payment (“ARP”) and then goes on to define the latter expression. So when I refer to the ARP, that means the Balancing Payment.
8. The definition of the ARP, over which the whole dispute turns, is outwardly uncomplicated:
“23.4% of the price achieved for each Residential Unit in excess of the Minimum Guaranteed Residential Unit Value less the Costs and Incentives.”
9. This contains three more defined concepts. Residential Unit means a flat. The Minimum Guaranteed Residential Unit Value (“MGRUV”) means the Total Residential Land Value divided by the number of flats. And Costs and Incentives (“C & I”) mean the additional expense which Persimmon might have to incur to induce someone to buy a flat; for example, by providing fittings better than specification or paying legal expenses. Such payments are economically equivalent to a reduction in the price achieved.
10. Chartbrook says that the meaning of the definition is perfectly simple. You take the price achieved, deduct the MGRUV and the C & I and calculate 23.4% of the result. That gives you a figure for an individual flat which, together the figures for similar calculations on all the other flats, makes up the ARP or Balancing Payment. That and the Total Land Value is the price. On the agreed figures, that produces a Total Land Value of £4,683,565 and an ARP of £4,484,862, making £9,168,427 in all. The judge (Briggs J)  EWHC 409 (Ch) and a majority of the Court of Appeal (Tuckey and Rimer LJJ)  EWCA Civ 183 agreed [...] Lawrence Collins LJ, dissenting, held that Persimmon’s construction was correct.
14. There is no dispute that the principles on which a contract (or any other instrument or utterance) should be interpreted are those summarised by the House of Lords in Investors Compensation Scheme Ltd v West Bromwich Building Society  1 WLR 896, 912-913. They are well known and need not be repeated. It is agreed that the question is what a reasonable person having all the background knowledge which would have been available to the parties would have understood them to be using the language in the contract to mean. The House emphasised that “we do not easily accept that people have made linguistic mistakes, particularly in formal documents” (similar statements will be found in Bank of Credit and Commerce International SA v Ali  1 AC 251, 269, Kirin-Amgen Inc v Hoechst Marion Roussel Ltd  RPC 169, 186 and Jumbo King Ltd v Faithful Properties Ltd (1999) 2 HKCFAR 279, 296) but said that in some cases the context and background drove a court to the conclusion that “something must have gone wrong with the language". In such a case, the law did not require a court to attribute to the parties an intention which a reasonable person would not have understood them to have had.
15. It clearly requires a strong case to persuade the court that something must have gone wrong with the language and the judge and the majority of the Court of Appeal did not think that such a case had been made out. On the other hand, Lawrence Collins LJ thought it had. It is, I am afraid, not unusual that an interpretation which does not strike one person as sufficiently irrational to justify a conclusion that there has been a linguistic mistake will seem commercially absurd to another: compare the Kirin-Amgen case  RPC 169 at pp. 189-190. Such a division of opinion occurred in the Investors Compensation Scheme case itself. The subtleties of language are such that no judicial guidelines or statements of principle can prevent it from sometimes happening. It is fortunately rare because most draftsmen of formal documents think about what they are saying and use language with care. But this appears to be an exceptional case in which the drafting was careless and no one noticed.
16. I agree with the dissenting opinion of Lawrence Collins LJ because I think that to interpret the definition of ARP in accordance with ordinary rules of syntax makes no commercial sense. [...] The term “Minimum Guaranteed Residential Unit Value", defined by reference to Total Residential Land Value, strongly suggests that this was to be a guaranteed minimum payment for the land value in respect of an individual flat. A guaranteed minimum payment connotes the possibility of a larger payment which, depending upon some contingency, may or may not fall due. Hence the term “Additional Residential Payment". The element of contingency is reinforced by paragraph 3.3 of the Sixth Schedule, which speaks of the “date of payment if any of the Balancing Payment.” (My emphasis).
17. The judge declined to regard the terms Total Land Value and Minimum Guaranteed Residential Unit Value as indicative of an intention that MGRUV was to be the minimum Chartbrook would receive as the land value of a flat because both terms were defined expressions. They might just as well have been algebraic symbols. Indeed they might, and I strongly suspect that if they had been, they would have made it clear that the parties were intending to give effect to Persimmon’s construction. But the contract does not use algebraic symbols. It uses labels. The words used as labels are seldom arbitrary. They are usually chosen as a distillation of the meaning or purpose of a concept intended to be more precisely stated in the definition. In such cases the language of the defined expression may help to elucidate ambiguities in the definition or other parts of the agreement: compare Birmingham City Council v Walker  2 AC 262, 268. I therefore consider that Lawrence Collins LJ was right to take into account the connotations of contingency to be derived from the defined terms.
18. On Chartbrook’s construction, there is virtually no element of contingency at all. ARP is payable in every case in which the flat sells for more than £53,438. Chartbrook submits that is still a contingency. Who could tell whether or not the market for flats in Wandsworth might not collapse? In the Court of Appeal, Rimer LJ accepted that submission. He said that the “relevant language", i.e. the language of contingency, was “strictly consistent also with Chartbrook’s construction.”
19. My Lords, I cannot believe that any rational parties who wished to make provision for such a catastrophic fall in the housing market (itself an unlikely assumption) would have adopted so precise a sum to represent their estimate of what might happen. Why £53,438? That was the agreed minimum figure for that part of the value of a flat attributable to the land which Chartbrook was selling. It was clearly based upon a careful and precise estimate of current market prices and building costs. But how could this figure have been appropriate as a minimum expected sale price of the entire flat at some future date? If the parties were wanting to guess at some extraordinary fall in the market against which Chartbrook was to be protected, why £53,438? Why not £50,000 or £60,000, or £100,000? A figure chosen to represent someone’s fears about a possible collapse in the market could only have been based upon wild speculation, not the kind of calculation which produces a figure like £53,438. That figure cannot have been meant to play the part in the calculation which Chartbrook’s construction assigns to it. It must have been intended to function as a minimum land value, not a minimum sale price. To compare it with the realised sale price would not be comparing like with like.
20. It is of course true that the fact that a contract may appear to be unduly favourable to one of the parties is not a sufficient reason for supposing that it does not mean what it says. The reasonable addressee of the instrument has not been privy to the negotiations and cannot tell whether a provision favourable to one side was not in exchange for some concession elsewhere or simply a bad bargain. But the striking feature of this case is not merely that the provisions as interpreted by the judge and the Court of Appeal are favourable to Chartbrook. It is that they make the structure and language of the various provisions of Schedule 6 appear arbitrary and irrational, when it is possible for the concepts employed by the parties (MGRUV, C & I etc) to be combined in a rational way.
21. I therefore think that Lawrence Collins LJ was right in saying that ARP must mean the amount by which 23.4% of the achieved price exceeds the MGRUV. I do not think that it is necessary to undertake the exercise of comparing this language with that of the definition in order to see how much use of red ink is involved. When the language used in an instrument gives rise to difficulties of construction, the process of interpretation does not require one to formulate some alternative form of words which approximates as closely as possible to that of the parties. It is to decide what a reasonable person would have understood the parties to have meant by using the language which they did. The fact that the court might have to express that meaning in language quite different from that used by the parties (“12th January” instead of “13th January” in Mannai Investment Co Ltd v Eagle Star Life Assurance Co Ltd  AC 749; “any claim sounding in rescission (whether for undue influence or otherwise)” instead of “any claim (whether sounding in rescission for undue influence or otherwise)” in Investors Compensation Scheme Ltd v West Bromwich Building Society  1 WLR 896) is no reason for not giving effect to what they appear to have meant.
27. If your Lordships agree with this conclusion about the construction of the contract, the appeal must be allowed. There is no need to say anything more. But Persimmon advanced two alternative arguments of very considerable general importance and I think it is appropriate that your Lordships should deal with them. The first was that (contrary to the unanimous opinion of the judge and the Court of Appeal) the House should take into account the pre-contractual negotiations, which in the opinion of Lawrence Collins LJ (at paragraph 132), were determinative confirmation of Persimmon’s argument on construction. The second was that the judge and the Court of Appeal had misunderstood the principles upon which rectification may be decreed and that if Persimmon had failed on construction, the agreement should have been rectified.
28. The rule that pre-contractual negotiations are inadmissible was clearly reaffirmed by this House in Prenn v Simmonds  1 WLR 1381, where Lord Wilberforce said (at p. 1384) that earlier authorities “contain little to encourage, and much to discourage, evidence of negotiation or of the parties’ subjective intentions.” It is clear that the rule of inadmissibility has been established for a very long time. In Inglis v John Buttery & Co (1878) 3 App Cas 552, 577 Lord Blackburn said that Lord Justice Clerk Moncreiff (at (1877) 4 R 58, 64) had laid down a principle which was nearly accurate but not quite when he said that in all mercantile contracts “whether they be clear and distinct or the reverse, the Court is entitled to be placed in the position in which the parties stood before they signed". The only qualification Lord Blackburn made was to reject Lord Moncreiff’s view that the Court was entitled to look at the pre-contractual negotiations because unless one did so, one could not be fully in the position in which the parties had been.
29. Instead, Lord Blackburn preferred (at p. 577) the opinion of Lord Gifford ((1877) 4 R 58, 69-70):
“Now, I think it is quite fixed - and no more wholesome or salutary rule relative to written contracts can be devised - that where parties agree to embody, and do actually embody, their contract in a formal written deed, then in determining what the contract really was and really meant, a Court must look to the formal deed and to that deed alone. This is only carrying out the will of the parties. The only meaning of adjusting a formal contract is, that the formal contract shall supersede all loose and preliminary negotiations - that there shall be no room for misunderstandings which may often arise, and which do constantly arise, in the course of long, and it may be desultory conversations, or in the course of correspondence or negotiations during which the parties are often widely at issue as to what they will insist on and what they will concede. The very purpose of a formal contract is to put an end to the disputes which would inevitably arise if the matter were left upon verbal negotiations or upon mixed communings partly consisting of letters and partly of conversations. The written contract is that which is to be appealed to by both parties, however different it may be from their previous demands or stipulations, whether contained in letters or in verbal conversation. There can be no doubt that this is the general rule, and I think the general rule, strictly and with peculiar appropriateness applies to the present case.”
30. To allow evidence of pre-contractual negotiations to be used in aid of construction would therefore require the House to depart from a long and consistent line of authority, the binding force of which has frequently been acknowledged: see Bank of Scotland v Dunedin Property Investment Co Ltd 1998 SC 657, 665 (“well-established and salutary", per Lord President Rodger; Alexiou v Campbell  UKPC 11 (“vouched by…compelling authorities", per Lord Bingham of Cornhill.) The House is nevertheless invited to do so, on the ground that the rule is illogical and prevents a court from, as the Lord Justice Clerk in Inglis v John Buttery & Co (1878) 3 App Cas 552 said, putting itself in the position of the parties and ascertaining their true intent.
31. In Prenn v Simmonds  1 WLR 1381, 1384 Lord Wilberforce said by way of justification of the rule:
“The reason for not admitting evidence of these exchanges is not a technical one or even mainly one of convenience, (though the attempt to admit it did greatly prolong the case and add to its expense). It is simply that such evidence is unhelpful. By the nature of things, where negotiations are difficult, the parties’ positions, with each passing letter, are changing and until the final agreement, though converging, still divergent. It is only the final document which records a consensus. If the previous documents use different expressions, how does construction of those expressions, itself a doubtful process, help on the construction of the contractual words? If the same expressions are used, nothing is gained by looking back: indeed, something may be lost since the relevant surrounding circumstances may be different. And at this stage there is no consensus of the parties to appeal to. It may be said that previous documents may be looked at to explain the aims of the parties. In a limited sense this is true: the commercial, or business object, of the transaction, objectively ascertained, may be a surrounding fact. Cardozo J. thought so in the Utica Bank case. And if it can be shown that one interpretation completely frustrates that object, to the extent of rendering the contract futile, that may be a strong argument for an alternative interpretation, if that can reasonably be found. But beyond that it may be difficult to go: it may be a matter of degree, or of judgment, how far one interpretation, or another, gives effect to a common intention: the parties, indeed, may be pursuing that intention with differing emphasis, and hoping to achieve it to an extent which may differ, and in different ways. The words used may, and often do, represent a formula which means different things to each side, yet may be accepted because that is the only way to get ‘agreement’ and in the hope that disputes will not arise. The only course then can be to try to ascertain the ‘natural’ meaning. Far more, and indeed totally, dangerous is it to admit evidence of one party’s objective - even if this is known to the other party. However strongly pursued this may be, the other party may only be willing to give it partial recognition, and in a world of give and take, men often have to be satisfied with less than they want. So, again, it would be a matter of speculation how far the common intention was that the particular objective should be realised.”
32. Critics of the rule, such as Thomas J in New Zealand (Yoshimoto v Canterbury Golf International Ltd  1 NZLR 523, 538-549) Professor David McLauchlan (“Contract Interpretation: What is it About?” (2009) 31:5 Sydney Law Review 5-51) and Lord Nicholls of Birkenhead (“My Kingdom for a Horse: The Meaning of Words” (2005) 121 LQR 577-591) point out that although all this may usually be true, in some cases it will not. Among the dirt of aspirations, proposals and counter-proposals there may gleam the gold of a genuine consensus on some aspect of the transaction expressed in terms which would influence an objective observer in construing the language used by the parties in their final agreement. Why should court deny itself the assistance of this material in deciding what the parties must be taken to have meant? Mr Christopher Nugee QC, who appeared for Persimmon, went so far as to say that in saying that such evidence was unhelpful, Lord Wilberforce was not only providing a justification for the rule but delimiting its extent. It should apply only in cases in which the pre-contractual negotiations are actually irrelevant. If they do assist a court in deciding what an objective observer would have construed the contract to mean, they should be admitted. I cannot accept this submission. It is clear from what Lord Wilberforce said and the authorities upon which he relied that the exclusionary rule is not qualified in this way. There is no need for a special rule to exclude irrelevant evidence.
33. I do however accept that it would not be inconsistent with the English objective theory of contractual interpretation to admit evidence of previous communications between the parties as part of the background which may throw light upon what they meant by the language they used. The general rule, as I said in Bank of Credit and Commerce International SA v Ali  1 AC 251, 269, is that there are no conceptual limits to what can properly be regarded as background. Prima facie, therefore, the negotiations are potentially relevant background. They may be inadmissible simply because they are irrelevant to the question which the court has to decide, namely, what the parties would reasonably be taken to have meant by the language which they finally adopted to express their agreement. For the reasons given by Lord Wilberforce, that will usually be the case. But not always. In exceptional cases, as Lord Nicholls has forcibly argued, a rule that prior negotiations are always inadmissible will prevent the court from giving effect to what a reasonable man in the position of the parties would have taken them to have meant. Of course judges may disagree over whether in a particular case such evidence is helpful or not. In Yoshimoto v Canterbury Golf International Ltd  1 NZLR 523. Thomas J thought he had found gold in the negotiations but the Privy Council said it was only dirt. As I have said, there is nothing unusual or surprising about such differences of opinion. In principle, however, I would accept that previous negotiations may be relevant.
34. It therefore follows that while it is true that, as Lord Wilberforce said, inadmissibility is normally based in irrelevance, there will be cases in which it can be justified only on pragmatic grounds. I must consider these grounds, which have been explored in detail in the literature and on the whole rejected by academic writers but supported by some practitioners.
35. The first is that the admission of pre-contractual negotiations would create greater uncertainty of outcome in disputes over interpretation and add to the cost of advice, litigation or arbitration. Everyone engaged in the exercise would have to read the correspondence and statements would have to be taken from those who took part in oral negotiations. Not only would this be time-consuming and expensive but the scope for disagreement over whether the material affected the construction of the agreement (as in the Yoshimoto case) would be considerably increased. As against this, it is said that when a dispute over construction is litigated, evidence of the pre-contractual negotiations is almost invariably tendered in support of an alternative claim for rectification (as in Prenn v Simmonds and in this case) or an argument based on estoppel by convention or some alleged exception to the exclusionary rule. Even if such an alternative claim does not succeed, the judge will have read and possibly been influenced by the evidence. The rule therefore achieves little in saving costs and its abolition would restore some intellectual honesty to the judicial approach to interpretation.
36. There is certainly a view in the profession that the less one has to resort to any form of background in aid of interpretation, the better. The document should so far as possible speak for itself. As Popham CJ said in the Countess of Rutland’s Case (1604) 5 Co Rep 25, 25b, 26a:
“it would be inconvenient, that matters in writing made by advice and on consideration, and which finally import the certain truth of the agreement of the parties should be controlled by averment of the parties to be proved by the uncertain testimony of slippery memory.”
37. I do not think that these opinions can be dismissed as merely based upon the fallacy that words have inherent or “available” meanings, rather than being used by people to express meanings, although some of the arguments advanced in support might suggest this. It reflects what may be a sound practical intuition that the law of contract is an institution designed to enforce promises with a high degree of predictability and that the more one allows conventional meanings or syntax to be displaced by inferences drawn from background, the less predictable the outcome is likely to be. In this respect, it is interesting to consider the reaction to the statement of principle in Investors Compensation Scheme Ltd v West Bromwich Building Society  1 WLR 896,912-913, which was viewed with alarm by some distinguished commercial lawyers as having greatly increased the quantity of background material which courts or arbitrators would be invited to consider: see Lord Bingham’s recent paper (“A New Thing Under the Sun: The Interpretation of Contract and the ICS Decision” (2008) 12 Edinburgh LR 374-390) and Spigelmann CJ, “From Text to Contract: Contemporary Contractual Interpretation” (2007) 81 ALJ 322. As Lord Bingham pointed out, there was little in that statement of principle which could not be found in earlier authorities. The only points it decided that might have been thought in the least controversial were, first, that it was not necessary to find an “ambiguity” before one could have any regard to background and, secondly, that the meaning which the parties would reasonably be taken to have intended could be given effect despite the fact that it was not, according to conventional usage, an “available” meaning of the words or syntax which they had actually used.
38. Like Lord Bingham, I rather doubt whether the ICS case produced a dramatic increase in the amount of material produced by way of background for the purposes of contractual interpretation. But pre-contractual negotiations seem to me capable of raising practical questions different from those created by other forms of background. Whereas the surrounding circumstances are, by definition, objective facts, which will usually be uncontroversial, statements in the course of pre-contractual negotiations will be drenched in subjectivity and may, if oral, be very much in dispute. It is often not easy to distinguish between those statements which (if they were made at all) merely reflect the aspirations of one or other of the parties and those which embody at least a provisional consensus which may throw light on the meaning of the contract which was eventually concluded. But the imprecision of the line between negotiation and provisional agreement is the very reason why in every case of dispute over interpretation, one or other of the parties is likely to require a court or arbitrator to take the course of negotiations into account. Your Lordships’ experience in the analogous case of resort to statements in Hansard under the rule in Pepper v Hart  AC 593 suggests that such evidence will be produced in any case in which there is the remotest chance that it may be accepted and that even these cases will be only the tip of a mountain of discarded but expensive investigation. Pepper v Hart has also encouraged ministers and others to make statements in the hope of influencing the construction which the courts will give to a statute and it is possible that negotiating parties will be encouraged to improve the bundle of correspondence with similar statements.
39. Supporters of the admissibility of pre-contractual negotiations draw attention to the fact that Continental legal systems seem to have little difficulty in taking them into account. Both the Unidroit Principles of International Commercial Contracts (1994 and 2004 revision) and the Principles of European Contract Law (1999) provide that in ascertaining the “common intention of the parties", regard shall be had to prior negotiations: articles 4.3 and 5.102 respectively. The same is true of the United Nations Convention on Contracts for the International Sale of Goods (1980). But these instruments reflect the French philosophy of contractual interpretation, which is altogether different from that of English law. As Professor Catherine Valcke explains in an illuminating article (“On Comparing French and English Contract Law: Insights from Social Contract Theory”) (16 January 2009), French law regards the intentions of the parties as a pure question of subjective fact, their volonté psychologique, uninfluenced by any rules of law. It follows that any evidence of what they said or did, whether to each other or to third parties, may be relevant to establishing what their intentions actually were. There is in French law a sharp distinction between the ascertainment of their intentions and the application of legal rules which may, in the interests of fairness to other parties or otherwise, limit the extent to which those intentions are given effect. English law, on the other hand, mixes up the ascertainment of intention with the rules of law by depersonalising the contracting parties and asking, not what their intentions actually were, but what a reasonable outside observer would have taken them to be. One cannot in my opinion simply transpose rules based on one philosophy of contractual interpretation to another, or assume that the practical effect of admitting such evidence under the English system of civil procedure will be the same as that under a Continental system.
41. The conclusion I would reach is that there is no clearly established case for departing from the exclusionary rule. The rule may well mean, as Lord Nicholls has argued, that parties are sometimes held bound by a contract in terms which, upon a full investigation of the course of negotiations, a reasonable observer would not have taken them to have intended. But a system which sometimes allows this to happen may be justified in the more general interest of economy and predictability in obtaining advice and adjudicating disputes. It is, after all, usually possible to avoid surprises by carefully reading the documents before signing them and there are the safety nets of rectification and estoppel by convention. Your Lordships do not have the material on which to form a view. It is possible that empirical study (for example, by the Law Commission) may show that the alleged disadvantages of admissibility are not in practice very significant or that they are outweighed by the advantages of doing more precise justice in exceptional cases or falling into line with international conventions. But the determination of where the balance of advantage lies is not in my opinion suitable for judicial decision. Your Lordships are being asked to depart from a rule which has been in existence for many years and several times affirmed by the House. There is power to do so under the Practice Statement (Judicial Precedent)  1 WLR 1234. But that power was intended, as Lord Reid said in R v National Insurance Comrs, Ex p Hudson  AC 944, 966, to be applied only in a small number of cases in which previous decisions of the House were “thought to be impeding the proper development of the law or to have led to results which were unjust or contrary to public policy". I do not think that anyone can be confident that this is true of the exclusionary rule.
42. The rule excludes evidence of what was said or done during the course of negotiating the agreement for the purpose of drawing inferences about what the contract meant. It does not exclude the use of such evidence for other purposes: for example, to establish that a fact which may be relevant as background was known to the parties, or to support a claim for rectification or estoppel. These are not exceptions to the rule. They operate outside it.
43. There is however a group of cases in which judges have found an exception to the exclusionary rule and your Lordships will have to decide whether such an exception can be justified. The leading case is the decision of Kerr J the Karen Oltmann (Partenreederei MS Karen Oltmann v Scarsdale Shipping Co Ltd  2 Lloyd’s Rep 708. This concerned a time charter for 2 years (14 days more or less in charterers’option) which contained a break clause:
“Charterers to have the option to redeliver the vessel after 12 months’ trading subject to giving three months’ notice".
44. The issue was whether “after 12 months’ trading” meant that the break clause could be operated only at the end of the first year or at any time during the second year. The judge said that he was entitled to look at telexes by which the fixture was negotiated in which the parties discussed various lengths of break clauses and were clearly using the word “after” to mean “on the expiry of” rather than “at any time after the expiry of". He justified the admissibility of this evidence on the following principle (p 712):
“If a contract contains words which, in their context, are fairly capable of bearing more than one meaning, and if it is alleged that the parties have in effect negotiated on an agreed basis that the words bore only one of the two possible meanings, then it is permissible for the court to examine the extrinsic evidence relied upon to see whether the parties have in fact used the words in question in one sense only, so that they have in effect given their own dictionary meaning to the words as the result of their common intention. Such cases would not support a claim for rectification of the contract, because the choice of words in the contract did not result from any mistake. The words used in the contract would ex hypothesi reflect the meaning which both parties intended.”
45. In his judgment in this case, Lawrence Collins LJ said of this principle (in paragraph 121) that he doubted whether it differed in any material respect from admitting evidence of prior negotiations in construing a contract. Indeed, the case is frequently cited as an example of an exception which undermines the rule: see for example Professor McLauchlan, “Contract Interpretation: What is It About?” (2009) 31:5 Sydney Law Review 5-51. It is true that evidence may always be adduced that the parties habitually used words in an unconventional sense in order to support an argument that words in a contract should bear a similar unconventional meaning. This is the “private dictionary” principle, which is akin to the principle by which a linguistic usage in a trade or among a religious sect may be proved: compare Shore v Wilson (1842) 9 Cl & F 355. For this purpose it does not matter whether the evidence of usage by the parties was in the course of negotiations or on any other occasion. It is simply evidence of the linguistic usage which they had in common. But the telexes in the Karen Oltmann did not evidence any unconventional usage. There was no private dictionary. The case involved a choice between two perfectly conventional meanings of the word “after” in a particular context. In my opinion Lawrence Collins LJ was right in saying that the admission of the evidence infringed the exclusionary rule. It is perhaps significant that the evidence merely confirmed the meaning which Kerr J, as an experienced commercial judge, would in any case have given to the clause.
46. What would have been the position if Kerr J had thought that, without the evidence of the telexes, he would have construed the clause in the opposite sense? He said that rectification would not be available because “The words used in the contract would ex hypothesi reflect the meaning which both parties intended.” I do not understand this, because, on this hypothesis, the telexes would show that the words (as construed by the judge) did not reflect the meaning which both parties intended. And it is generally accepted that Brightman J was right in Re Butlin’s Settlement Trusts  Ch 251 in holding that rectification is available not only when the parties intended to use different words but also when they mistakenly thought their words bore a different meaning.
47. On its facts, the Karen Oltmann was in my opinion an illegitimate extension of the “private dictionary” principle which, taken to its logical conclusion, would destroy the exclusionary rule and any practical advantages which it may have. There are two legitimate safety devices which will in most cases prevent the exclusionary rule from causing injustice. But they have to be specifically pleaded and clearly established. One is rectification. The other is estoppel by convention, which has been developed since the decision in the Karen Oltmann: see Amalgamated Investment & Property Co. Ltd. v. Texas Commerce International Bank Ltd.  QB 84. If the parties have negotiated an agreement upon some common assumption, which may include an assumption that certain words will bear a certain meaning, they may be estopped from contending that the words should be given a different meaning. Both of these remedies lie outside the exclusionary rule, since they start from the premise that, as a matter of construction, the agreement does not have the meaning for which the party seeking rectification or raising an estoppel contends.