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ASCII stands for American Standard Code for Information Interchange. It is a 7-bit coded character set with 128 code positions, including both control characters and graphic characters. The name describes a standard for representing information so different information-processing and communication systems can exchange it.
What ASCII is—and how many characters it has
ASCII assigns code positions to a defined set of characters and controls. Because its standard form uses 7 bits, it has 27, or 128, possible positions. Those positions include more than letters and digits: the set also contains control characters that perform functions affecting data or equipment.
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ASCII is the name of the character set, not a claim about how many bits a computer must use to store or transmit each character. A system can place a 7-bit ASCII value inside a larger storage unit.
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Control characters and graphic characters
The standard distinguishes two broad kinds of characters:
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- Control characters are defined by what they do in context. The 1977 standard describes one as a character whose occurrence can initiate, modify, or stop an action affecting the recording, processing, transmission, or interpretation of data. NUL (Null), CR (Carriage Return), and LF (Line Feed) are examples.
- Graphic characters have visual representations normally written, printed, or displayed. Their meaning in a particular application can depend on how users employ them.
Controls do not all serve the same purpose. The standard groups them by function, including communication controls, format effectors, and information separators. Nor does the character-set definition by itself determine how every modern application will interpret a control character.
Why ASCII is sometimes described alongside 8-bit bytes
ASCII itself is a 7-bit character set. In RFC 20, a 1969 recommendation for network interchange, Vint Cerf proposed embedding each 7-bit ASCII value in an 8-bit byte with the high-order bit set to zero. That is a byte convention for the RFC’s network context; it does not make ASCII an 8-bit character set.
This distinction matters when reading descriptions that call ASCII “8-bit.” They may be referring to a byte used to carry an ASCII value, or to a different 8-bit encoding—not to the standard ASCII repertoire itself.
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How ASCII became a standard
ASCII emerged when computer systems already used different codes. NIST’s historical account describes IBM’s 8-bit EBCDIC and Control Data’s 6-bit FIELDATA among the approaches in use. Work on a common code began in 1961 through a committee chartered by the American Standards Association. The organization later became USASI and then ANSI.
The dates in the standards history refer to different milestones rather than a single publication event. A NIST-hosted foreword says ASCII was first issued in 1963, then published with minor revisions as X3.4-1967 and X3.4-1968. NIST’s historical account describes the set as having evolved to 128 characters by 1967 and says USASI published the 7-bit ASCII standard in 1968. These accounts distinguish initial issuance, revisions, and the later publication milestone.
ASCII and US-ASCII in Internet contexts
ASCII is the usual name for the standard. US-ASCII is an Internet character-set name listed by IANA. The registry references ANSI X3.4-1986 for the US-ASCII entry; its page was last updated on 2026-10-02. RFC 20, by contrast, is a period document that refers to the X3.4-1968 table and describes a network-use convention.
What ASCII does not cover
ASCII defines a finite repertoire of 128 code positions. It should not be treated as a character set for every language or as a representation of all modern text. The term identifies this particular standard, not every encoding that uses bytes or every system that represents characters.
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