The international standard ISO 10646 defines the "Universal Character Set (UCS)". UCS contains all characters of all other character set standards. It also guarantees "round-trip compatibility", i.e., conversion tables can be built such that no information is lost when a string is converted from any other encoding to UCS and back.
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 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 backwards compatibility with ASCII processing software and UTF-16 for the backwards 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".
Some code points in UCS have been assigned to "combining characters". These are similar to the non-spacing accent keys on a typewriter. A combining character just adds an accent to the previous character. The most important accented characters have codes of their own in UCS, however, the combining character mechanism allows us to add accents and other diacritical marks to any character. The combining characters always follow the character which they modify. For example, the German character Umlaut-A ("Latin capital letter A with diaeresis") can either be represented by the precomposed UCS code 0x00c4, or alternatively as the combination of a normal "Latin capital letter A" followed by a "combining diaeresis": 0x0041 0x0308.
Combining characters are essential for instance for encoding the Thai script or for mathematical typesetting and users of the International Phonetic Alphabet.
As not all systems are expected to support advanced mechanisms like combining characters, ISO 10646-1 specifies the following three implementation levels of UCS: