UCS contains the characters required to represent practically all known languages. This includes not only the Latin, Greek, Cyrillic, Hebrew, Arabic, Armenian, and Georgian scripts, but also Chinese, Japanese and Korean Han ideographs as well as scripts such as Hiragana, Katakana, Hangul, Devanagari, Bengali, Gurmukhi, Gujarati, Oriya, Tamil, Telugu, Kannada, Malayalam, Thai, Lao, Khmer, Bopomofo, Tibetan, Runic, Ethiopic, Canadian Syllabics, Cherokee, Mongolian, Ogham, Myanmar, Sinhala, Thaana, Yi, and others. For scripts not yet covered, research on how to best encode them for computer usage is still going on and they will be added eventually. This might eventually include not only Hieroglyphs and various historic Indo-European languages, but even some selected artistic scripts such as Tengwar, Cirth, and Klingon. UCS also covers a large number of graphical, typographical, mathematical, and scientific symbols, including those provided by TeX, Postscript, APL, MS-DOS, MS-Windows, Macintosh, OCR fonts, as well as many word processing and publishing systems, and more are being added.
The UCS standard (ISO 10646) describes a 31-bit character set architecture consisting of 128 24-bit groups, each divided into 256 16-bit planes made up of 256 8-bit rows with 256 column positions, one for each character. Part 1 of the standard (ISO 10646-1) defines the first 65534 code positions (0x0000 to 0xfffd), which form the Basic Multilingual Plane (BMP), that is plane 0 in group 0. Part 2 of the standard (ISO 10646-2) adds characters to group 0 outside the BMP in several supplementary planes in the range 0x10000 to 0x10ffff. There are no plans to add characters beyond 0x10ffff to the standard, therefore of the entire code space, only a small fraction of group 0 will ever be actually used in the foreseeable future. The BMP contains all characters found in the commonly used other character sets. The supplemental planes added by ISO 10646-2 cover only more exotic characters for special scientific, dictionary printing, publishing industry, higher-level protocol and enthusiast needs.
The representation of each UCS character as a 2-byte word is referred to as the UCS-2 form (only for BMP characters), whereas UCS-4 is the representation of each character by a 4-byte word. In addition, there exist two encoding forms UTF-8 for backward compatibility with ASCII processing software and UTF-16 for the backward-compatible handling of non-BMP characters up to 0x10ffff by UCS-2 software.
The UCS characters 0x0000 to 0x007f are identical to those of the classic US-ASCII character set and the characters in the range 0x0000 to 0x00ff are identical to those in ISO 8859-1 (Latin-1).
Combining characters are essential for instance for encoding the Thai script or for mathematical typesetting and users of the International Phonetic Alphabet.
The Unicode 3.0 Standard published by the Unicode Consortium contains exactly the UCS Basic Multilingual Plane at implementation level 3, as described in ISO 10646-1:2000. Unicode 3.1 added the supplemental planes of ISO 10646-2. The Unicode standard and technical reports published by the Unicode Consortium provide much additional information on the semantics and recommended usages of various characters. They provide guidelines and algorithms for editing, sorting, comparing, normalizing, converting, and displaying Unicode strings.
UCS/Unicode can be used just like ASCII in input/output streams, terminal communication, plaintext files, filenames, and environment variables in the ASCII compatible UTF-8 multibyte encoding. To signal the use of UTF-8 as the character encoding to all applications, a suitable locale has to be selected via environment variables (e.g., "LANG=en_GB.UTF-8").
The nl_langinfo(CODESET) function returns the name of the selected encoding. Library functions such as wctomb(3) and mbsrtowcs(3) can be used to transform the internal wchar_t characters and strings into the system character encoding and back and wcwidth(3) tells, how many positions (0-2) the cursor is advanced by the output of a character.
Two other planes are reserved for private usage, plane 15 (Supplementary Private Use Area-A, range 0xf0000 to 0xffffd) and plane 16 (Supplementary Private Use Area-B, range 0x100000 to 0x10fffd).