The shannon (symbol Sh) is a unit of information and of entropy defined by IEC 80000-13. One shannon is the information content of an event when the probability of that event occurring is one half. It is also the entropy of a system with two equiprobable states. If a message is made of a sequence of bits, with all possible bit strings equally likely, the message's information content expressed in shannons is equal to the number of bits in the sequence. For this and historical reasons, a shannon is more commonly known as a bit, despite that "bit" is also used as a unit of data (or of computer storage, equal to 1/8 of a byte).