System of symbols and rules used for expressing information according to an unvarying rule for replacing a piece of information from one system, such as a letter, word, or phrase, with an arbitrarily selected equivalent in another system. Substitution ciphers are similar to codes except that the rule for replacing the information is known only to the transmitter and the intended recipient of the information. Binary code and other machine languages used in digital computers are examples of codes. Elaborate commercial codes were developed during the early 20th century (seeJean M.E. Baudot, Samuel F. B. Morse). In recent years more advanced codes have been developed to accommodate computer data and satellite communications. See alsoASCII, cryptography.