Chain codes are a form of digit sequence which have error-correcting properties when suitably arranged, and methods of finding the best code for any particular application are detailed.A particular equipment is described in outline which would be suitable for transmitting the contents of one computer store over a telephone line and into another store.The paper shows that chain codes with less than 2ndigits in the major chain can be effectively used to transmit 2nmessages with a specified amount of error correction.