[For more current case annotated texts by this author, see Bernstein & Lookofsky, Understanding the CISG in Europe, 2d ed. (2003) and Lookofsky, Understanding the CISG in the USA, 2d ed. (2004).]
87. By virtue of CISG Article 9, commercial custom and usage become part of the international contract of sale. Article 9 provides as follows:
2. The parties are considered, unless otherwise agreed, to have impliedly made applicable to their contract or its formation a usage of which the parties knew or ought to have known and which in international trade is widely known to, and regularly observed by, parties to contracts of the type involved in the particular trade concerned.'[page 57]
A. Express Agreement and Inter Partes Course of Dealing
88. The first part of Paragraph 1 covers the seemingly rare situation where the parties have actually agreed to be bound by a given trade usage; indeed, virtually any agreement between the parties (including, of course, an express contractual provision) takes precedence over the otherwise applicable CISG supplementary rule.
In addition, Paragraph 1 also regulates a more commercially significant group of cases where the parties have established a practice between themselves.
B. Implied Incorporation of Commercial Usage
89. Paragraph 2 of Article 9 lays down the requirements for the implied incorporation of a given usage in the particular international trade. To be applicable, the usage must be one which the parties knew or ought to have known and which in international trade is widely known to, and regularly observed by, parties to contracts of the type involved in the particular trade concerned.
90. An international custom which fulfils the Article 9 criteria takes precedence over the Convention's supplementary rules, i.e., both the contract formation rules of Part II as well as the gap-filling rights and obligations as defined in Part III. Thus, whether a buyer who fails initially to discover a non-conformity can later allege breach may well depend on an established practice between the parties or a (well known) trade usage regarding the nature of a buyer's duty to inspect within the area of trade concerned (caveat emptor). Trade usage may also affect, e.g. an international seller's right to claim interest for late payment at the rate specified in standard business terms.