If we agree to something in dealing with the other party, the law of contracts is involved. For example, suppose we want to have a new machine built and installed by one of the firms which builds special equipment, the law of contracts forms an unseen backdrop for our attempts to acquire the machine. It is unlikely that we would have occasion to use these legal principles in a court action; but it might happen, and the best way to prevent the occurrence is to observe the following principles when the contract is formed and carried out.