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Plane (Unicode)
(詳細はUnicode standard, a plane is a continuous group of 65,536 (= 216) code points. There are 17 planes, identified by the numbers 0 to 16decimal, which corresponds with the possible values 00–10hexadecimal of the first two positions in six position format (''hhhhhh''). The planes above plane 0 (the Basic Multilingual Plane), that is, planes 1–16, are called “supplementary planes”,〔(Unicode Consortium Glossary—Supplementary Planes )〕 or humorously known as “astral planes”. As of Unicode version 8.0, six of the planes have assigned code points (characters), and four are named.
The 17 planes can accommodate 1,114,112 code points, of which 2048 are surrogates, 66 are non-characters, and 137,468 are reserved for private use, leaving 974,530 for public assignment. The limit (which is not a power of 2) is due to the design of UTF-16, and is the maximum value that can be encoded by it.〔The four bits wwww in the high surrogate represents the (Unicode plane − 1). Unicode plane = wwww + 1. The highest value wwww can represent is 1111binary = Fhex = 15decimal. Hence plane (15 + 1)=16 is the highest plane a surrogate pair can represent. Hence 10 FFFFhex is the highest code point a surrogate pair can represent. See Table 3.5 "UTF-16 Bit Distribution" in the Unicode Standard http://www.unicode.org/versions/Unicode6.0.0/UnicodeStandard-6.0.pdf〕 UTF-8 was designed with a much larger limit of 231 code points (32768 planes), and can encode 221 code points (32 planes) even if limited to 4 bytes.〔See Table 3.6 "UTF-8 Bit Distribution" in the Unicode Standard http://www.unicode.org/versions/Unicode6.0.0/UnicodeStandard-6.0.pdf〕 However, the Unicode Consortium has stated that the limit will never be raised.〔(【引用サイトリンク】title=Unicode Character Encoding Stability Policy )〕
Planes are further subdivided into Unicode blocks, which unlike planes, do not have a fixed size. The 262 blocks defined in Unicode 8.0 cover 24 percent of the possible code point space, and range in size from a minimum of 16 code points (eleven 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 every known current and ancient writing system.〔(Unicode roadmaps )〕
==Overview==
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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