a. The provisions in the CISG on the conformity of goods to the sales contract are included in Article 35. This article is in three sections, of which the first, the conformity of the goods to the terms of the contract, consists of the primary rule for assessing lack of conformity. Only where the parties have not agreed otherwise will the secondary rule in Article 35(2) apply, which provides a number of positively expressed assumptions about the contractual requirements for the goods. Finally, Article 35(3) contains an exception to the seller's liability for some lack of conformity of goods, where the buyer knew or could not have been unaware of the lack of conformity.

b. Unlike the CISG, the UNIDROIT Principles and the PECL cover not only sales contracts but also other types of contracts (e.g., contracts of service). To provide maximum flexibility and to facilitate future development of the two sets of principles, the fundamental rules regarding the conformity of performance have been described in more general terms. In Article 7.1.1. of the UNIDROIT Principles, non-performance is defined as a failure by a party to perform any of its obligations under the contract, including defective performance or late performance. The same approach is taken in PECL; see Article 8:101(1) with Comment A. Although the UNIDROIT Principles and the PECL do not have any rules that directly resemble CISG Article 35, several articles may be used to interpret or supplement it.