The teletype code is a data transmission codein which five bits each represent one character. With five bits, a total of only 32 (`2^5`) different characters can be represented and thus not the entire alphabet with 26 letters and the ten digits from 0 to 9.
The representable character set was extended by special control characters. With these control characters the character set is switched from digits to letters. The 5-bit character 11111 is used for switching to letters, the control character 11011 is used for switching to digits. The bit combination 00000 is not used.
Thus 29 5-bit combinations are available which can be used twice, i.e. a total of 58 digits, letters, characters and control characters. Of the 58 possible characters, 52 are used.
The actual 5-bit code was developed by Baudot in 1871 and corresponds to teletype code No. 1, CCITT code No. 1. The teletype code still used today in its extended form with 52 letters, digits and characters was developed by Donald Murray and is called teletype code No.2. This code is often referred to as the Baudot code and is used in teletypes.