In the Unicode standard, a plane is a continuous group of 65,536 (= 216) code points. There are 17 planes, identified by the numbers 0 to 16decimal, which corresponds with the possible values 00–10hexadecimal of the first two positions in six position format (hhhhhh). The planes above plane 0 (the Basic Multilingual Plane), that is, planes 1–16, are called “supplementary planes”, or humorously known as “astral planes”. As of version 7.0, six of the planes have assigned code points (characters), and four are named.
Currently, about ten percent of the potential space is used. Furthermore, ranges of characters have been tentatively mapped out for every current and ancient writing system (script) the Unicode Consortium has been able to identify. Even if previously unknown scripts with tens of thousands of characters are discovered, the limit of 1,114,112 code points is unlikely to be reached in the near future. The Unicode Consortium has stated that the limit will never be changed.
The odd-looking code points limit (it is not a power of 2) is due to the design of UTF-16. In UTF-16 a "surrogate pair" of two 16-bit words is used to encode 220 code points in the planes 1 to 16, in addition to the use of single code unit to encode plane 0. It is not due to UTF-8, which was designed with a limit of 231 code points (32768 planes), and can encode 221 code points (32 planes) even if limited to 4 bytes.
|Unicode planes and used code point ranges|
|Plane 0||Plane 1||Plane 2||Planes 3–13||Plane 14||Planes 15–16|
|Basic Multilingual Plane||Supplementary Multilingual Plane||Supplementary Ideographic Plane||unassigned||Supplementary Special-purpose Plane||Supplementary Private Use Area|
|BMP||SMP||SIP||—||SSP||S PUA A/B|
The first plane, plane 0, the Basic Multilingual Plane (BMP), is where most characters have been assigned so far. The BMP contains characters for almost all modern languages, and a large number of special characters. A primary objective for the BMP is to support the unification of prior character sets as well as characters for writing. Most of the allocated code points in the BMP are used to encode Chinese, Japanese, and Korean (CJK) characters.
The High Surrogates (U+D800–U+DBFF) and Low Surrogate (U+DC00–U+DFFF) codes are reserved for encoding non-BMP characters in UTF-16 by using a pair of 16-bit codes: one High Surrogate and one Low Surrogate. A single surrogate code point will never be assigned a character.
As of Unicode 7.0[update], the BMP comprises the following blocks:
Plane 1, the Supplementary Multilingual Plane (SMP), contains historic scripts such as Linear B, Egyptian hieroglyphs, and cuneiform scripts; historic and modern musical notation; mathematical alphanumerics; Emoji and other pictographic sets; reform orthographies like Shavian and Deseret; and game symbols for playing cards, Mah Jongg, and dominoes.
As of Unicode 7.0[update], the SMP comprises the following blocks:
Plane 2, the Supplementary Ideographic Plane (SIP), is used for CJK Ideographs, mostly CJK Unified Ideographs, that were not included in earlier character encoding standards.
As of Unicode 6.1[update], the SIP comprises the following blocks:
Planes 3 to 13: No characters have yet been assigned to Planes 3 through 13. Plane 3 is tentatively named the Tertiary Ideographic Plane, but as of version 7.0 there are no characters assigned to it. It is reserved for Oracle Bone script, Bronze Script, Small Seal Script, additional CJK unified ideographs, and other historic ideographic scripts.
It is not anticipated that all these planes will be used in the foreseeable future, given the total sizes of the known writing systems left to be encoded. The number of possible symbol characters that could arise outside of the context of writing systems is potentially huge. At the moment, these 11 planes out of 17 are unused.
Plane 14 (E in hexadecimal), the Supplementary Special-purpose Plane (SSP), currently contains non-graphical characters. The first block is for deprecated language tag characters for use when language cannot be indicated through other protocols (such as the xml:lang attribute in XML). The other block contains glyph variation selectors to indicate an alternate glyph for a character that cannot be determined by context.
As of Unicode 6.1[update], the SSP comprises the following blocks:
The two planes 15 and 16, called Supplementary Private Use Area-A and -B are available for character assignment by parties outside the ISO and the Unicode Consortium. They are used by fonts internally to refer to auxiliary glyphs, for example, ligatures and building blocks for other glyphs. Such characters will have limited interoperability. Software and fonts that support Unicode will not necessarily support character assignments by other parties.