Contracts very often stipulate the procedures by which they may be altered. Those procedures can be very simple or very elaborate.
An example of a simple variations clause, specifying that variations must be in writing, is as follows:
This Agreement may not be varied except by a written document signed by or on behalf of each of the parties.
The purpose of this kind of clause is to ensure that contractual terms are not varied accidentally or informally, and further to prevent an unscrupulous party from falsely claiming that a contract has been varied on the telephone or during a face-to-face conversation, where the only evidence available is the contradictory accounts of the two people involved.
Some types of contracts - for example complex services agreements such as outsourcing agreements - provide for very specific procedures for varying the specification of services/products and other contract terms. In some cases, there may be different procedures for different kinds of variation.