Source: http://www.cisg.law.pace.edu/cisg/text/ziegel18.html
Timestamp: 2019-04-25 18:03:30+00:00

Document:
1. The first sentence of Art. 18 possibly goes further than the common law since it fails to take into account a particular mode or place of acceptance prescribed by the offeror. The common law rule is that the offeror is master of his offer and hence is entitled to require such mode and place of acceptance as he deems fit. However, nowadays the courts are relucant to find that an indicated mode of acceptance was intended to be the exclusive mode. Cf. Waddams, op. cit., pp. 63-64, and UCC 2-206(1).
2. Paragraph (2) adopts the reception theory of acceptance common to many civil law jurisdictions, and not the postal theory of the common law. Both rules are defensible and merchants in common law jurisdictions should have no difficulty in adjusting to the reception rule. In any event it is not uncommon for an offer to stipulate that an acceptance is not effective until it reaches the offeror.
3. There is a dearth of Anglo-Canadian authority with respect to the circumstances in which an offer may be accepted by performance or part performance where the offer itself is silent with respect to the question. Art. 18(3) is not too helpful since it merely refers us back to the offer or the parties' practices or the usages of the trade in which they are engaged. UCC 2-206 and section 29 of the American Second Restatement on the Law of Contracts (Tent. Draft) permit acceptance "in any manner and by any medium reasonable in the circumstance" including, it would seem, performance of the requested act. UCC 2-206(1)(b) further provides that: "an order or other offer to buy goods for prompt or current shipment shall be construed as inviting acceptance either by a prompt promise to ship by the prompt or current shipment of conforming ... goods, ...".
The [Secretariat] Commentary (p. 64) to the draft convention indicates that its authors envisage a similar result under art. 18(3).
(a) If the offeree is entitled to accept by performance must he give notice of his election to the offeror where the offeror is not otherwise likely to learn promptly of the election? UCC 2-206(1)(b) says yes, and presumably the same conclusion would be reached under the good faith provisions of art. 7.
(b) Can the beginning of a requested performance constitute an acceptance? UCC 2-206(2) clearly acknowledges the possibility provided prompt notice of the acceptance is given to the offeror. To the same effect, see Restatement on Contracts 2d, s. 63. Semble, the Anglo-Canadian cases have so far only considered the problem in the context of unilateral contracts. See e.g., Errington v. Errington (1952) 1 K.B. 290 (C.A.); Sloan v. Union Oil of Canada Co. Ltd. (1955) 4 D.L.R. 664 (B.C.). The CISG [Secretariat] Commentary (p. 64) recognizes that commencement of requested performance may be a sufficient act of acceptance. The conclusion is supported by the language of art. 18(3), which merely requires assent by performing "an act". This is different from requiring full performance.

References: Art. 18
 Art. 18
 art. 18
 art. 7
 v. 
 v. 
 art. 18