Article 9 CISG explains the order in which commercial customs are to be taken into consideration for the purposes of interpreting and supplementing the parties' contract of sale. Accordingly, the parties are bound by practices existing between them. Article 9(1) CISG states further that these practices may supplement the terms of the contract, but that the binding force of such practices is an expression of the principle of good faith, e.g.for example, the parties may have adopted a certain means of sending the numerous communications required under the CISG. Consequently, a repeated waiver of rights by one party may establish a subsequent contractual commitment.[194] Secondly, a word must be said to agreed usages, whose terms the parties are considered to have incorporated into their contract. Due to the contractual agreement and as to the incorporation of the usage, it is irrelevant whether the actual sales transaction falls within the scope of that very usage or whether the requirements for the existence of an international usage are fulfilled, provided the usage exists.[195]
The borderline of such a general, agreed implementation often merges with that of codified rules, such as INCOTERMS.[197]Article 9 CISG remains silent as to the perception of mercantile terms and INCOTERMS although its predecessor Article 9(3) a Uniform Law on the International Sale of Goods (ULIS) provided that common expressions, clauses or forms are to be interpreted according to the meaning normally accorded to them by the relevant trade sectors. Even though such standard clauses and forms are now common in international trade, the rule was considered to be superfluous and was therefore eliminated. Article 8(2) CISG was introduced instead. Therefore, even on an international level, the question arises whether INCOTERMS must be contractually incorporated into the contract to be used as a means of interpretation, or if such an agreement is unnecessary.