Baudot code in the context of Teletypewriter

In computing and telecommunications, a character is the encoded representation of a natural language character (including letter, numeral and punctuation), whitespace (space or tab), or a control character (controls computer hardware that consumes character-based data). A sequence of characters is called a string.

Some character encoding systems represent each character using a fixed number of bits whereas other systems use varying sizes. Various fixed-length sizes were used for now obsolete systems such as the six-bit character code, the five-bit Baudot code and even 4-bit systems (with only 16 possible values). The more modern ASCII system uses the 8-bit byte for each character. Today, the Unicode-based UTF-8 encoding uses a varying number of byte-sized code units to define a code point which combine to encode a character.

A teleprinter (teletypewriter, teletype or TTY) is an electromechanical device used to send and receive typed messages through various communications channels, in both point-to-point and point-to-multipoint configurations.

Initially, from 1887 at the earliest, teleprinters were used in telegraphy. Electrical telegraphy had been developed decades earlier in the late 1830s and 1840s, then using simpler Morse key equipment and telegraph operators. The introduction of teleprinters automated much of this work and eventually largely replaced skilled operators versed in Morse code with typists and machines communicating faster via Baudot code.

A bell character (sometimes bell code) is a device control code originally sent to ring a small electromechanical bell on tickers and other teleprinters and teletypewriters to alert operators at the other end of the line, often of an incoming message. Though tickers punched the bell codes into their tapes, printers generally do not print a character when the bell code is received. Bell codes are usually represented by the label BEL. They have been used since 1870 (initially in the Baudot code).

To maintain backward compatibility, video display terminals (VDTs) that replaced teletypewriters included speakers or buzzers to perform the same function, as did the personal computers that followed. Modern terminal emulators often integrate the warnings to the desktop environment (e.g., the macOS Terminal will play the system warning sound) and also often offer a silent visual bell feature that flashes the terminal window briefly.

In computing, an escape sequence is a sequence of characters that has a special semantic meaning based on an established convention that specifies an escape character prefix in addition to the syntax of the rest of the text of a sequence. A convention can define any particular character code as a sequence prefix. Some conventions use a normal, printable character such as backslash (\) or ampersand (&). Others use a non-printable (a.k.a. control) character such as ASCII escape.

Escape sequences date back at least to the 1874 Baudot code.

Jean-Maurice-Émile Baudot (French: [eˈmil boˈdo]; 11 September 1845 – 28 March 1903), French telegraph engineer and inventor of the first means of digital communication Baudot code, was one of the pioneers of telecommunications. He invented a multiplexed printing telegraph system that used his code and allowed multiple transmissions over a single line. The baud unit was named after him.

A telegraph code is one of the character encodings used to transmit information by telegraphy. Morse code is the best-known such code. Telegraphy usually refers to the electrical telegraph, but telegraph systems using the optical telegraph were in use before that. A code consists of a number of code points, each corresponding to a letter of the alphabet, a numeral, or some other character. In codes intended for machines rather than humans, code points for control characters, such as carriage return, are required to control the operation of the mechanism. Each code point is made up of a number of elements arranged in a unique way for that character. There are usually two types of element (a binary code), but more element types were employed in some codes not intended for machines. For instance, American Morse code had about five elements, rather than the two (dot and dash) of International Morse Code.

Codes meant for human interpretation were designed so that the characters that occurred most often had the fewest elements in the corresponding code point. For instance, Morse code for E, the most common letter in English, is a single dot ( ▄ ), whereas Q is  ▄▄▄ ▄▄▄ ▄ ▄▄▄ . These arrangements meant the message could be sent more quickly and it would take longer for the operator to become fatigued. Telegraphs were always operated by humans until late in the 19th century. When automated telegraph messages came in, codes with variable-length code points were inconvenient for machine design of the period. Instead, codes with a fixed length were used. The first of these was the Baudot code, a five-bit code. Baudot has only enough code points to print in upper case. Later codes had more bits (ASCII has seven) so that both upper and lower case could be printed. Beyond the telegraph age, modern computers require a very large number of code points (Unicode has 21 bits) so that multiple languages and alphabets (character sets) can be handled without having to change the character encoding. Modern computers can easily handle variable-length codes such as UTF-8 and UTF-16 which have now become ubiquitous.