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 very last code point in Unicode is the last code point in plane 16, U+10FFFF. As of Unicode version 13.0, seven of the planes have assigned code points (characters), and five are named.

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

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 can 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 308 blocks defined in Unicode 13.0 cover 26% of the possible code point space, and range in size from a minimum of 16 code points (fifteen 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 as of Unicode version 13.0
Plane Allocated code points[note 1] Assigned characters[note 2]
 0 BMP 65,472 55,503
 1 SMP 24,704 22,279
2 SIP 60,912 60,866
3 TIP 4,944 4,939
14 SSP 368 337
15 SPUA-A 65,536
16 SPUA-B 65,536
Totals 287,472 143,924
  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.

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.

65,472 of the 65,536 code points in this plane have been allocated to a Unicode block, leaving just 64 code points in unallocated ranges (48 code points at 0870..089F and 16 code points at 2FE0..2FEF).

As of Unicode 13.0, the BMP comprises the following 163 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, and Adlam. Symbols and notations include historic and modern musical notation; mathematical alphanumerics; shorthands; Emoji and other pictographic sets; and game symbols for playing cards, Mah Jongg, and dominoes.

As of Unicode 13.0, the SMP comprises the following 134 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 13.0, the SIP comprises the following six 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, Bronze Script, and Small Seal Script.[6]

As of Unicode 13.0, the TIP comprises the following block:

Unassigned planes

Planes 4 to 13 (planes 4 to D in hexadecimal): No characters have yet been assigned 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), the Supplementary Special-purpose Plane (SSP), currently contains non-graphical characters. The first block is for special use tag characters. The other block contains glyph variation selectors to indicate an alternate glyph for a character that cannot be determined by context.

As of Unicode 13.0, the SSP comprises the following two blocks:

  • Tags (E0000–E007F)
  • Variation Selectors Supplement (E0100–E01EF)

Private Use Area planes

The two planes 15 and 16 (planes F and 10 in hexadecimal), are designated as "private use planes". They contain blocks called Supplementary Private Use Area-A (PUA-A) and -B (PUA-B), Private Use Areas, which are available for character assignment by parties outside the ISO and the Unicode Consortium. They may be 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. 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. Unicode roadmaps
  5. Unicode, Inc. "Announcing The Unicode® Standard, Version 13.0".
  6. "Proposed New Characters: The Pipeline". www.unicode.org.
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