In the Unicode standard, planes are groups of numerical values (code points) that point to specific characters. Unicode code points are logically divided into 17 planes, each with 65,536 (= 216) code points. Planes are 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). As of version 6.0, six of these planes have assigned code points (characters), and 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.[1] While Unicode may eventually need to use another of the spare 11 planes for ideographic characters, other planes remain. 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 limit will never be changed.
The odd-looking 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 1 to 16, in addition to the use of single words 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.
Sometimes, the terms “astral plane” and “astral characters” are used informally to refer to the planes above the Basic Multilingual Plane (planes 1–16) and their characters.[2]
Contents |
| Unicode planes and code point (character) ranges | ||||||||
|---|---|---|---|---|---|---|---|---|
| Basic | Supplementary | |||||||
| Plane 0: Basic Multilingual Plane |
Plane 1: Supplementary Multilingual Plane |
Plane 2: Supplementary Ideographic Plane |
Planes 3–13: Unassigned |
Plane 14: Supplementary Special-purpose Plane |
Planes 15–16: Supplementary Private Use Area |
|||
| 0000–FFFF | 10000–1FFFF | 20000–2FFFF | 30000–DFFFF | E0000–EFFFF | F0000–10FFFF | |||
| BMP | SMP | SIP | — | SSP | S PUA A/B | |||
| 0000–0FFF 1000–1FFF 2000–2FFF 3000–3FFF 4000–4FFF 5000–5FFF 6000–6FFF 7000–7FFF |
8000–8FFF 9000–9FFF A000–AFFF B000–BFFF C000–CFFF D000–DFFF E000–EFFF F000–FFFF |
10000–10FFF 11000–11FFF 12000–12FFF 13000–13FFF 16000–16FFF |
1B000–1BFFF 1D000–1DFFF 1F000–1FFFF |
20000–20FFF 21000–21FFF 22000–22FFF 23000–23FFF 24000–24FFF 25000–25FFF 26000–26FFF 27000–27FFF |
28000–28FFF 29000–29FFF 2A000–2AFFF 2B000–2BFFF 2F000–2FFFF |
E0000–E0FFF | 15: PUA-A F0000–FFFFF 16: PUA-B 100000–10FFFF |
|
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 6.0[update], the BMP comprises the following blocks:
|
|
|
Plane 1, the Supplementary Multilingual Plane (SMP), is mostly used for historic scripts such as Linear B, and is also used for musical and mathematical symbols.
As of Unicode 6.0[update], the SMP comprises the following blocks:
|
|
|
Plane 2, the Supplementary Ideographic Plane (SIP), is used for Unified Han (CJK) Ideographs that were mostly not included in earlier character encoding standards.
As of Unicode 6.0[update], the SIP comprises the following blocks:
Planes 3 to 13: Unicode has not yet assigned any characters to Planes 3 through 13. Plane 3 is tentatively named the Tertiary Ideographic Plane, but has no characters assigned yet as of version 6.0. It is reserved for Oracle Bone script, Bronze Script, Small Seal Script, additional CJK unified ideographs, and other historic ideographic scripts.[3]
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 6.0[update], the SSP comprises the following blocks:
The two planes 15 and 16, called Supplementary Private Use Area-A and -B (or simply Private Use Area (PUA)) 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.