Unicode planes

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). The very last code point in Unicode is the last code point in plane 16, U+10FFFF. Plane 0 is the Basic Multilingual Plane (BMP), which contains most commonly-used characters. The higher planes 1 through 16 are called "supplementary planes". As of Unicode version 10.0, six of the planes have assigned code points (characters), and four 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.. 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 if limited to 4 bytes.

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 280 blocks defined in Unicode 10.0 cover 25% of the possible code point space, and range in size from a minimum of 16 code points (thirteen 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.

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.

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