In the Unicode standard, a plane is a contiguous 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]
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[update], the BMP comprises the following 164 blocks:
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[update], the SIP comprises the following seven blocks:
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[update]:
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).
