One common situation arises when the offeror stipulates the mode of acceptance (e.g., return mail, fax, or carrier pigeon). If the offeree uses the stipulated mode, then the acceptance is deemed effective when sent. Even though the offeror has no knowledge of the acceptance at that moment, the contract has been formed. Moreover, according to the Restatement, Section 60, if the offeror says that the offer can be accepted only by the specified mode, that mode must be used. (It is said that “the offeror is the master of the offer.”)
If the offeror specifies no particular mode, then acceptance is effective when transmitted, as long as the offeree uses a reasonable method of acceptance. It is implied that the offeree can use the same means used by the offeror or a means of communication customary to the industry.