ISO IEC 646-1991 PDF

Full title and description

Information technology — ISO 7-bit coded character set for information interchange (ISO/IEC 646:1991). This international standard specifies a 7-bit coded character set of 128 control and graphic characters (letters, digits, punctuation and control codes) intended for interchange of information using Latin-script alphabets.

Abstract

ISO/IEC 646:1991 defines the set of 128 code positions for a 7‑bit coded character set, describes the invariant/basic character set and the International Reference Version (IRV), and documents the positions that are left available for national variants. The 1991 edition aligns the IRV with US‑ASCII and updates a small number of symbol allocations (notably the currency sign/dollar sign change in the IRV).

General information

• Status: Published (International Standard; edition 3, 1991).
• Publication date: December 1991 (edition 3, published 16 December 1991 / cataloged 1991-12).
• Publisher: ISO (joint ISO/IEC publication; national adoption by bodies such as BSI, ANSI/INCITS, SIS).
• ICS / categories: 35.040.10 (coding of character sets / information interchange).
• Edition / version: Edition 3 (1991).
• Number of pages: 15 pages in the ISO international edition (note: some national-adoption documents or national publications may show different pagination).

Scope

The standard specifies the graphic and control characters for a 7‑bit coded character set intended for interchange of information in Latin‑script alphabets. It identifies an invariant/basic subset that must remain unchanged across national variants, defines the International Reference Version (IRV) for use when no national variant is required, and documents which code positions are available for national substitutions. It is focused on 7‑bit encodings rather than later 8‑bit or multibyte systems.

Key topics and requirements

• Definition of 128 code positions: 0x00–0x7F including control and graphic characters.
• Specification of the invariant/basic character set (characters that must not be changed in national variants).
• Definition and description of the International Reference Version (IRV) and its alignment with ASCII in the 1991 edition.
• Designation of code positions allowed for national use and guidance on national variants (deprecated practice for new systems).
• Provision of default control character set where applicable.

Typical use and users

Used historically by system implementers, device/terminal manufacturers, communications engineers, archivists and any organizations exchanging plain text across systems that required a common 7‑bit repertoire. Today it is primarily of interest for legacy system support, data conversion, archival interpretation of older files and for understanding the historical basis of ASCII and national variants.

Related standards

Closely related to ECMA‑6 and US‑ASCII; it forms the basis for later standards such as ISO/IEC 4873 (8‑bit extensions), the ISO/IEC 8859 series (8‑bit single‑byte coded graphic character sets), ISO/IEC 10646 and Unicode (multilingual / universal character repertoires), and various national ISO‑IR registrations. It is also related to ITU‑T T.50 and several national character set standards (e.g., ISO‑IR registrations, JIS X mappings).

Keywords

7‑bit, ISO 646, ISO/IEC 646, ASCII, International Reference Version (IRV), invariant set, character set, national variants, control characters, Latin script, legacy encoding.

FAQ

Q: What is this standard?

A: ISO/IEC 646:1991 is the third edition (1991) international standard that specifies a 7‑bit coded character set for information interchange — effectively the ISO specification of the basic ASCII repertoire and the rules for national variants.

Q: What does it cover?

A: It covers the mapping of 128 code positions to control and graphic characters, defines the invariant/basic set and the International Reference Version (IRV), and indicates which positions may be replaced in national variants. It does not define 8‑bit extensions or multibyte encodings.

Q: Who typically uses it?

A: Historically used by implementers of terminals, printers, communication systems and data interchange protocols. Today it is mainly used by professionals handling legacy data, archivists, standards historians and anyone requiring precise interpretation of older 7‑bit encoded text.

Q: Is it current or superseded?

A: The 1991 edition is an official ISO/IEC publication (edition 3). Functionally, the world has largely moved to 8‑bit encodings and Unicode/ISO/IEC 10646 for modern interchange; ISO/IEC 646 remains relevant for legacy compatibility but newer standards are recommended for contemporary systems.

Q: Is it part of a series?

A: Yes — it is part of the family of character set standards and is closely associated with ECMA‑6, ISO/IEC 4873 (8‑bit structure), ISO/IEC 8859 series, and the broader Unicode/ISO/IEC 10646 efforts that followed to provide extended and universal repertoires.

Q: What are the key keywords?

A: 7‑bit, ASCII, IRV, invariant set, national variant, control characters, Latin script, ISO/IEC 646.