Plane (Unicode)

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”,[1] or humorously known as “astral planes”. As of Unicode version 7.0, six of the planes have assigned code points (characters), and four are named.

The 17 planes can accommodate 1,114,112 code points, a limit which is unlikely to be reached in the foreseeable future even if previously unknown scripts with tens of thousands of characters are discovered. The odd-looking code points limit (which 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 units to encode the 216 points of plane 0.[2] This limit is not shared by 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.[3] However, the Unicode Consortium has stated that the current limit will never be changed.[4]

Planes are further subdivided into Unicode blocks, which unlike planes, do not have a fixed size. The 252 blocks defined in Unicode 7.0 cover 23 percent of the possible code point space, and range in size from a minimum of 16 code points (eleven blocks) to a maximum of 65,536 code points (Supplementary Private Use Area-A and -B, which constitute the entirety of planes 15 and 16). For future usage, ranges of characters have been tentatively mapped out for every known current and ancient writing system.[5]

Overview

Unicode planes and used code point ranges
Basic Supplementary
Plane 0 Plane 1 Plane 2 Planes 3–13 Plane 14 Planes 15–16
0000–​FFFF 10000–​1FFFF 20000–​2FFFF 30000–​DFFFF E0000–​EFFFF F0000–​10FFFF
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

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
1E000–​1EFFF
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

Assigned characters as of Unicode version 7.0
Plane Allocated code points[note 1] Assigned characters[note 2]
 0 BMP 65,312 55,056
 1 SMP 11,936 10,004
2 SIP 47,648 47,624
14 SSP 368 337
15 PUA-A 65,536
16 PUA-B 65,536
Totals 256,336 113,021
  1. ↑ Code points which have been allocated to a Unicode block.
  2. ↑ The total number of graphic, format and control characters (i.e., excluding private-use characters, noncharacters and surrogate code points).

Basic Multilingual Plane

A map of the Basic Multilingual Plane. Each numbered box represents 256 code points.
(As of Unicode Standard version 6.0)

The first plane, plane 0, the Basic Multilingual Plane (BMP) contains characters for almost all modern languages, and a large number of symbols. A primary objective for the BMP is to support the unification of prior character sets as well as characters for writing. Most of the assigned 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.

65,312 of the 65,536 code points in this plane have been allocated to a Unicode block, leaving just 224 non-allocated code points (fourteen 16-character segments).


As of Unicode 7.0, the BMP comprises the following 159 blocks:

Supplementary Multilingual Plane

A map of the Supplementary Multilingual Plane. Each numbered box represents 256 code points.
(As of Unicode Standard version 6.2)

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, the SMP comprises the following 85 blocks:

Supplementary Ideographic Plane

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, the SIP comprises the following four blocks:

Unassigned planes

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.[6] It is reserved for Oracle Bone script, Bronze Script, Small Seal Script, additional CJK unified ideographs, and other historic ideographic scripts.[7]

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.

Supplementary Special-purpose Plane

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, the SSP comprises the following two blocks:

Private Use Area planes

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.

References

  1. ↑ Unicode Consortium Glossary—Supplementary Planes
  2. ↑ The four bits wwww in the high surrogate represents the (Unicode plane − 1). Unicode plane = wwww + 1. The highest value wwww can represent is 1111binary = Fhex = 15decimal. Hence plane (15 + 1)=16 is the highest plane a surrogate pair can represent. Hence 10 FFFFhex is the highest code point a surrogate pair can represent. See Table 3.5 "UTF-16 Bit Distribution" in the Unicode Standard http://www.unicode.org/versions/Unicode6.0.0/UnicodeStandard-6.0.pdf
  3. ↑ See Table 3.6 "UTF-8 Bit Distribution" in the Unicode Standard http://www.unicode.org/versions/Unicode6.0.0/UnicodeStandard-6.0.pdf
  4. ↑ "Unicode Character Encoding Stability Policy". Retrieved 27 September 2014.
  5. ↑ Unicode roadmaps
  6. ↑ "Unicode Data". Retrieved 11 July 2014.
  7. ↑ TIP Roadmap