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 16, which corresponds with the possible values 00–1016 of the first two positions in six position hexadecimal format (U+hhhhhh). Plane 0 is the Basic Multilingual Plane (BMP), which contains most commonly used characters. The higher planes 1 through 16 are called "supplementary planes".[1] The last code point in Unicode is the last code point in plane 16, U+10FFFF. As of Unicode version 15.1, five of the planes have assigned code points (characters), and seven are named.

The limit of 17 planes is due to UTF-16, which can encode 220 code points (16 planes) as pairs of words, plus the BMP as a single word.[2] UTF-8 was designed with a much larger limit of 231 (2,147,483,648) code points (32,768 planes), and would still be able to encode 221 (2,097,152) code points (32 planes) even under the current limit of 4 bytes.[3]

The 17 planes can accommodate 1,114,112 code points. Of these, 2,048 are surrogates (used to make the pairs in UTF-16), 66 are non-characters, and 137,468 are reserved for private use, leaving 974,530 for public assignment.

Planes are further subdivided into Unicode blocks, which, unlike planes, do not have a fixed size. The 328 blocks defined in Unicode 15.1 cover 26% of the possible code point space, and range in size from a minimum of 16 code points (sixteen 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 most known current and ancient writing systems.[4]

Overview

Assigned characters
Plane Allocated code points[note 1] version 15.0 Assigned characters
 0 BMP 65,520 55,639
 1 SMP 26,160 23,276
 2 SIP 61,536 61,495
 3 TIP 9,136 9,131
14 SSP 368 337
15 SPUA-A 65,536 0 (by definition)
16 SPUA-B 65,536 0 (by definition)
Totals 293,792 149,878
  1. Code points which have been allocated to a Unicode block.

Basic Multilingual Plane
A map of the Basic Multilingual Plane. Each numbered box represents 256 code points.

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 Surrogate (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.

65,520 of the 65,536 code points in this plane have been allocated to a Unicode block, leaving just 16 code points in a single unallocated range (2FE0..2FEF).

As of Unicode 15.1, the BMP comprises the following 164 blocks:

Supplementary Multilingual Plane
A map of the Supplementary Multilingual Plane. Each numbered box represents 256 code points.

Plane 1, the Supplementary Multilingual Plane (SMP), contains historic scripts (except CJK ideographic), and symbols and notation used within certain fields. Scripts include Linear B, Egyptian hieroglyphs, and cuneiform scripts. It also includes English reform orthographies like Shavian and Deseret, and some modern scripts like Osage, Warang Citi, Adlam, Wancho and Toto. Symbols and notations include historic and modern musical notation; mathematical alphanumerics; shorthands; Emoji and other pictographic sets; and game symbols for playing cards, mahjong, and dominoes.

As of Unicode 15.1, the SMP comprises the following 151 blocks:

Supplementary Ideographic Plane
A map of the Supplementary Ideographic Plane. Each numbered box represents 256 code points.

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 15.1, the SIP comprises the following seven blocks:

Tertiary Ideographic Plane
A map of the Tertiary Ideographic Plane. Each numbered box represents 256 code points.

Plane 3 is the Tertiary Ideographic Plane (TIP). CJK Unified Ideographs Extension G was added to the TIP in Unicode 13.0, released in March 2020.[5] It also is tentatively allocated for Oracle Bone script and Small Seal Script.[6]

As of Unicode 15.1, the TIP comprises the following two blocks:

Unassigned planes

Planes 4 to 13 (planes 4 to D in hexadecimal): No characters have yet been assigned, or proposed for assignment, to Planes 4 through 13.

Supplementary Special-purpose Plane
A map of the Supplementary Special-purpose Plane. Each numbered box represents 256 code points.

Plane 14 (E in hexadecimal) is designated as the Supplementary Special-purpose Plane (SSP). It comprises the following two blocks, as of Unicode 15.1:

Private Use Area Planes
"Private Use Plane" redirects here. For private aircraft, see commercial aviation, business aviation, general aviation, and civil aviation.

The two planes 15 and 16 (planes F and 10 in hexadecimal) each contain a "Private Use Area". They contain blocks named Supplementary Private Use Area-A (PUA-A) and -B (PUA-B). The Private Use Areas are available for use by parties outside ISO and Unicode (private character encoding).

References
  1. "Glossary". www.unicode.org. Retrieved 2021-09-27.
  2. See Table 3.5 "UTF-16 Bit Distribution" in the Unicode Standard https://www.unicode.org/versions/Unicode6.0.0/UnicodeStandard-6.0.pdf
  3. See Table 3.6 "UTF-8 Bit Distribution" in the Unicode Standard https://www.unicode.org/versions/Unicode6.0.0/UnicodeStandard-6.0.pdf
  4. "Roadmaps to Unicode". www.unicode.org. Retrieved 2021-09-27.
  5. "Announcing The Unicode Standard, Version 13.0".
  6. "Proposed New Characters: The Pipeline". www.unicode.org.
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