1. Indication of Assent
Art. 18(1)(1) provides for the general rule:
(1) A statement made by or other conduct of the offeree indicating assent to an offer is an acceptance.
Decisive in determining whether a statement or conduct of the offeree indicates assent is an interpretation pursuant to Art. 8 taking into account practices and usages under Art. 9. Examples include dispatching the goods, issuing payment, or taking delivery.
The second sentence of Art. 18(1) continues:
Silence or inactivity does not in itself amount to acceptance.
Important here is the term “in itself”. According to this qualification, silence can amount to acceptance if it is combined with other circumstances. Such circumstances can be factors listed in Art. 8(3) or practices or usages under Art. 9. Yet, the general rule contained in this provision makes it clear that the CISG does not recognize the legal notion of a commercial letter of confirmation in response to an oral conclusion of contract that – e.g. under German law – leads to a conclusion of contract with the terms set out in the letter if the recipient of the letter of confirmation does not object to its content. Where both parties, however, are from jurisdictions that know the legal notion of a commercial letter of confirmation such letter might have binding effect via recognition of the notion as an applicable trade usage in terms of Art. 9.
Under Art. 18(2)(1) effectiveness of an acceptance is analogous to the offer:
(2) An acceptance of an offer becomes effective at the moment the indication of assent reaches the offeror.
Art. 18(3) establishes an exception from this general rule:
(3) However, if, by virtue of the offer or as a result of practices which the parties have established between themselves or of usage, the offeree may indicate assent by performing an act, such as one relating to the dispatch of the goods or payment of the price, without notice to the offeror, the acceptance is effective at the moment the act is performed […].
The wording of the provision is clear in the regard that an act that would usually constitute conduct indicating assent in terms of Art. 18(1) still is necessary and merely the requirement of this act “reaching” the offeror is not required.