Plane (Unicode)

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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]