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This Article lays down the principle according to which the parties are in general bound by practices and usages which meet the requirements set forth in the Article. Furthermore, these same requirements must be met by practices and usages for them to be applicable in the cases and for the purposes expressly indicated in the Principles (see, for instance, Articles 2.1.6(3), 4.3, and 5.1.2).

A practice established between the parties to a particular contract is automatically binding, except where the parties have expressly excluded its application. Whether a particular practice can be deemed to be ‘established’ between the parties will naturally depend on the circumstances of the case, but behaviour on the occasion of only one previous transaction between the parties will not normally suffice.

By stating that the parties are bound by usages to which they have agreed, paragraph (1) of this Article merely applies the general principle of freedom of contract laid down in Article 1.1. Indeed, the parties may either negotiate all the terms of their contract, or for certain questions simply refer to other sources including usages. The parties may stipulate the application of any usage, including a usage developed within a trade sector to which neither party belongs, or a usage relating to a different type of contract. It is even conceivable that the parties will agree on the application of what sometimes misleadingly are called usages, i.e. a set of rules issued by a particular trade association under the title of ‘Usages’, but which only in part reflects established general lines of conduct.

Paragraph (2) lays down the criteria for the identification of usages applicable in the absence of a specific agreement by the parties. The fact that the usage must be ‘widely known to and regularly observed […] by parties in the particular trade concerned’ is a condition for the application of any usage, be it at international or merely at national or local level. The additional qualification ‘in international trade’ is intended to avoid usages developed for, and confined to, domestic transactions also being invoked in transactions with foreigners.

Only exceptionally may usages of a purely local or national origin be applied without any reference thereto by the parties. Thus, usages existing on certain commodity exchanges or at trade exhibitions or ports should be applicable provided that they are regularly followed with respect to foreigners as well. Another exception concerns the case of a businessperson who has already entered into a number of similar contracts in a foreign country and who should therefore be bound by the usages established within that country for such contracts.

A usage may be regularly observed by the generality of business people in a particular trade sector but its application in a given case may nevertheless be unreasonable. Reasons for this may be found in the particular conditions in which one or both parties operate and/or the atypical nature of the transaction. In such cases the usage will not be applied.

Both courses of dealing and usages, once they are applicable in a given case, prevail over conflicting provisions contained in the Principles. The reason for this is that they bind the parties as implied terms of the contract as a whole or of single statements or other conduct on the part of one of the parties. As such, they are superseded by any express term stipulated by the parties but, in the same way as the latter, they prevail over the Principles, the only exception being those provisions which are specifically declared to be of a mandatory character (see Comment 3 on Article 1.5).