History

From a five-key keyboard on a French telegraph line to the standard character set of the world’s telex network.

1. 1874 Émile Baudot, an engineer of the French telegraph administration, patents a printing telegraph. His operators enter each character as a chord on five keys, so every character takes five on/off units.
2. 1901 Donald Murray rearranges the code for a typewriter-style keyboard that punches paper tape. Frequent letters get patterns with few holes, which spares the punch mechanism, and control functions such as carriage return and line feed become characters of their own.
3. 1910s–20s Start-stop teleprinters spread. Each character is framed by a start and a stop element, so two machines stay in step without a shared clock.
4. 1932 The usual date for international adoption: the telegraph consultative committee that later became part of the ITU standardises the code as International Telegraph Alphabet No. 2. Baudot’s original code becomes known as ITA1.
5. 1930s Switched teleprinter services, later known as telex, begin; subscribers dial each other directly and exchange printed messages at 50 baud.
6. 1940s–50s Radioteletype carries ITA2 over shortwave, for example for news agencies and weather services.
7. 1963 The 7-bit ASCII code is published. With room for lower-case letters and many more signs, it and its international form, Alphabet No. 5, gradually replace ITA2 in computing.
8. 1988 The ITU publishes the current text of Recommendation S.1, which still defines ITA2.
9. Today Radio amateurs keep the code on the air, most visibly in radioteletype contests.

Why five units

Five units give 32 combinations: enough for the Latin alphabet plus a few controls, and short enough to send quickly by hand or by machine. The shift idea, reusing the same patterns for a second set of characters, kept the code at five units for decades and shaped the keyboards of every teleprinter that used it.

The name of the baud

The unit of signalling speed, the baud, is named after Émile Baudot. One baud is one signal element per second; a 50-baud telex line sends fifty elements, not fifty characters, every second.