Supplementary Special-purpose Plane

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_decimal, which corresponds with the possible values 00–10_hexadecimal of the first two positions in six position format (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", or humorously "astral planes". As of Unicode version 9.0, six of the planes have assigned code points (characters), and four are named.

The limit of 17 (which is not a power of 2) is due to the design of UTF-16, which can encode 16 supplementary planes and the BMP, to a maximum value of 0x10FFFF, the last code point in plane 16. The encoding scheme used by UTF-8 was designed with a much larger limit of 2³¹ code points (32,768 planes), and can encode 2²¹ code points (32 planes) even if limited to 4 bytes. Since Unicode limits the code points to the 17 planes that can be encoded by UTF-16, code points above 0x10FFFF are invalid in UTF-8 and UTF-32.

The 17 planes can accommodate 1,114,112 code points. Of these, 2,048 are surrogates, 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 273 blocks defined in Unicode 9.0 cover 24% of the possible code point space, and range in size from a minimum of 16 code points (twelve 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.

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