Principles of International Commercial Contracts, 1994 - UNIDROIT
Chapter 2 - Formation
Article 2.6 - Mode of Acceptance
(1) A statement made by or other conduct of the offeree indicating assent to an offer is an acceptance. Silence or inactivity does not in itself amount to acceptance.
(2) An acceptance of an offer becomes effective when the indication of assent reaches the offeror.
(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 without notice to the offeror, the acceptance is effective when the act is performed.