ISO 11940:1998

Overview

ISO 11940:1998 - "Information and documentation - Transliteration of Thai" defines a strict, completely reversible system for converting Thai script into Roman characters. Its purpose is machine‑readable, unambiguous Romanization (transliteration) that allows automatic transmission and exact reconstitution of Thai text by people or systems. The standard prioritizes reversibility over phonetic fidelity and uses diacritics, digraphs and ordering rules to represent Thai characters uniquely.

Key topics and technical requirements

• Reversible transliteration: Every Thai character is mapped to a unique Roman sequence so that retransliteration back to Thai is unambiguous.
• Romanization table: A normative mapping (code points) assigns each Thai character a corresponding sequence of Roman letters and combining marks (diacritics). For example, Ko Kai (U+0E01) → "k"; some characters map to letter + diacritic sequences (e.g., Kho Khai → k + macron + h as specified in the table).
• Diacritic and special symbols: Four special symbols (macron, macron‑below, dot‑below, horn) are used to distinguish Roman representations of different Thai characters.
• Character sequence rules: Thai multi‑level combining characters (uppermost, upper, lower) must be transliterated with a defined typing sequence - consonant first, then uppermost mark, then upper or lower level marks as required.
• Typing sequence for Roman output: If a Roman character uses diacritics, the base character is typed before the combining symbols (order defined for single or multiple special symbols).
• Capitalization rules: Capitals are reserved for initials in proper nouns; the standard warns against using capitals for transliteration of specific Thai characters so that proper and common nouns can still be distinguished.
• Normative reference: ISO/IEC 10646‑1 (UCS / Unicode baseline) is referenced for code positions.

Applications

• Library and bibliographic cataloguing, indexing and alphabetic intercalation where unambiguous back‑conversion to Thai is required.
• Data interchange, search and retrieval, digital archives, OCR post‑processing, geographic names transliteration (toponymy), and multilingual databases.
• Software localization, text‑processing tools and NLP pipelines that must preserve original Thai graphisms for retransliteration.

Who should use it

• Librarians, archivists and bibliographers responsible for catalogues.
• Standards bodies, localization and GIS teams handling Thai names.
• Software developers, database engineers and NLP researchers who require lossless, machine‑processable transliteration of Thai.

Related standards

• ISO/IEC 10646‑1 (Universal Multiple‑Octet Coded Character Set / Unicode) - normative reference for character code positions.
• ISO technical work on conversion of writing systems (ISO/TC 46, SC 2) - context for transliteration standards.

Keywords: ISO 11940, transliteration of Thai, Thai Romanization, reversible transliteration, Unicode, Romanization table, Thai script mapping.