This section is from the book "The Law Of Contracts", by William Herbert Page. Also available from Amazon: Commercial Contracts: A Practical Guide to Deals, Contracts, Agreements and Promises.
As in the case of the offer which upon acceptance will amount to a contract,1 the uncomniunicated intention of one party to waive rights has no legal consequence, and in order to amount to waiver such intention must be communicated to the adversary party. The means by which such intention is communicated is ordinarily immaterial. The party may communicate his intention by express words.2 If the authorized agent of an insurance company, when notified of a vacancy, informs the insured that his policy remains in full force, such notice operates as a waiver of a condition against vacancy.3 Far more frequently the intention to waive is communicated by the acts and conduct of the party who waives the right.4 The practical question which arises in cases of this sort is generally the nature of the acts or conduct by which such intention is shown, and of the extent to which, on the one hand, a mere declaration of an intent to waive is sufficient, or, on the other hand, of the necessity of action in reliance upon such declaration which would result in prejudice to the parties so acting if waiver were held not to exist.1