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.
Performance of a contract consists in doing the thing agreed to be done. A contract may he performed by both parties thereto; in which case it is said to be fully executed. The use of this word is inadvisable, since "executed" is used and with more accuracy, in speaking of the formation of a contract. A contract may be performed by one party, but not by the other. Such a contract is said to be executed on one side and executory on the other. The name "unilateral" is also given to contracts of this class. The use of this word is undesirable.