A“contract” can be defined roughly as an agreement that can be enforced by the courts or other governmental mechanisms. Roman and Anglo-American law share this notion so far as it goes. An obvious question, however, is how to tell which agreements rise to the level of contracts. In common law, this is a fairly simple issue in principle. Roughly, any seriously intended “agreement, upon a sufficient consideration, to do or not to do a particular thing” counts. “Consideration” here means simply the thing(s) you get in return for fulfilling the agreement. In Roman law, things are more complicated. To be legally enforceable, an agreement had to meet the description of one of several pre-defined types of contract. There were more than ten types, and these were in turn divided into groups in different ways (e.g., on the basis of who was under obligation or how the agreement was to be interpreted). In the rest of this chapter I will simplify the situation by treating only five of the most important types of contract.

The most important distinction is between contracts defined by their formalities, on the one hand, and by their content, on the other. Formalities are special words or actions that might be required in the making of a contract, such as writing down an agreement, signing that agreement, shaking hands, registration in some central archive, or even using the word “contract.”