[3]: 215 §13.6, 236 §4 The committee considered an eight-bit code, since eight bits (octets) would allow two four-bit patterns to efficiently encode two digits with binary-coded decimal.

[3]: 217 §c, 236 §5  Eight-bit machines (with octets as the native data type) that did not use parity checking typically set the eighth bit to 0.

[3]: 220, 236 8, 9)  The "space" character had to come before graphics to make sorting easier, so it became position 20hex;[3]: 237 §10  for the same reason, many special signs commonly used as separators were placed before digits.

The @ symbol was not used in continental Europe and the committee expected it would be replaced by an accented À in the French variation, so the @ was placed in position 40hex, right before the letter A.

These are codes intended to control peripheral devices (such as printers), or to provide meta-information about data streams, such as those stored on magnetic tape.

[35] Except for the control characters that prescribe elementary line-oriented formatting, ASCII does not define any mechanism for describing the structure or appearance of text within a document.

The ambiguity this caused was sometimes intentional, for example where a character would be used slightly differently on a terminal link than on a data stream, and sometimes accidental, for example the standard is unclear about the meaning of "delete".

Probably the most influential single device affecting the interpretation of these characters was the Teletype Model 33 ASR, which was a printing terminal with an available paper tape reader/punch option.

In particular, the Teletype Model 33 machine assignments for codes 17 (control-Q, DC1, also known as XON), 19 (control-S, DC3, also known as XOFF), and 127 (delete) became de facto standards.

The early Unix tty drivers, unlike some modern implementations, allowed only one character to be set to erase the previous character in canonical input processing (where a very simple line editor is available); this could be set to BS or DEL, but not both, resulting in recurring situations of ambiguity where users had to decide depending on what terminal they were using (shells that allow line editing, such as ksh, bash, and zsh, understand both).

The assumption that no key sent a BS character allowed Ctrl+H to be used for other purposes, such as the "help" prefix command in GNU Emacs.

They are usually in the form of a so-called "ANSI escape code" (often starting with a "Control Sequence Introducer", "CSI", "ESC [") from ECMA-48 (1972) and its successors.

In graphical user interface (GUI) and windowing systems, ESC generally causes an application to abort its current operation or to exit (terminate) altogether.

The inherent ambiguity of many control characters, combined with their historical usage, created problems when transferring "plain text" files between systems.

By the time so-called "glass TTYs" (later called CRTs or "dumb terminals") came along, the convention was so well established that backward compatibility necessitated continuing to follow it.

Until the introduction of PC DOS in 1981, IBM had no influence in this because their 1970s operating systems used EBCDIC encoding instead of ASCII, and they were oriented toward punch-card input and line printer output on which the concept of "carriage return" was meaningless.

On the other hand, the original Macintosh OS, Apple DOS, and ProDOS used carriage return (CR) alone as a line terminator; however, since Apple later replaced these obsolete operating systems with their Unix-based macOS (formerly named OS X) operating system, they now use line feed (LF) as well.

The Unix terminal driver uses the end-of-transmission character (EOT), also known as control-D, to indicate the end of a data stream.

[6][50] ASCII was first used commercially during 1963 as a seven-bit teleprinter code for American Telephone & Telegraph's TWX (TeletypeWriter eXchange) network.

[52] On March 11, 1968, US President Lyndon B. Johnson mandated that all computers purchased by the United States Federal Government support ASCII, stating:[53][54][55] I have also approved recommendations of the Secretary of Commerce [Luther H. Hodges] regarding standards for recording the Standard Code for Information Interchange on magnetic tapes and paper tapes when they are used in computer operations.

All computers and related equipment configurations brought into the Federal Government inventory on and after July 1, 1969, must have the capability to use the Standard Code for Information Interchange and the formats prescribed by the magnetic tape and paper tape standards when these media are used.ASCII was the most common character encoding on the World Wide Web until December 2007, when UTF-8 encoding surpassed it; UTF-8 is backward compatible with ASCII.

[56][57][58] As computer technology spread throughout the world, different standards bodies and corporations developed many variations of ASCII to facilitate the expression of non-English languages that used Roman-based alphabets.

From early in its development,[59] ASCII was intended to be just one of several national variants of an international character code standard.

Many other countries developed variants of ASCII to include non-English letters (e.g. é, ñ, ß, Ł), currency symbols (e.g. £, ¥), etc.

programmer using their national variant of ISO/IEC 646, rather than ASCII, had to write, and thus read, something such as instead of C trigraphs were created to solve this problem for ANSI C, although their late introduction and inconsistent implementation in compilers limited their use.

For some countries, 8-bit extensions of ASCII were developed that included support for characters used in local languages; for example, ISCII for India and VISCII for Vietnam.

In other cases, the extra bit was used for some other purpose, such as toggling inverse video; this approach was used by ATASCII, an extension of ASCII developed by Atari.

[62] Digital Equipment Corporation developed the Multinational Character Set (DEC-MCS) for use in the popular VT220 terminal as one of the first extensions designed more for international languages than for block graphics.

A popular further extension designed by Microsoft, Windows-1252 (often mislabeled as ISO-8859-1), added the typographic punctuation marks needed for traditional text printing.

[57] ISO/IEC 4873 introduced 32 additional control codes defined in the 80–9F hexadecimal range, as part of extending the 7-bit ASCII encoding to become an 8-bit system.