Plane (Unicode)

Plane (Unicode)

In the Unicode standard, planes are groups of numerical values (code points) that point to specific characters. Unicode code points are logically divided into 17 planes, each with 65,536 (= 216) code points. Planes are 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). As of version 6.1, six of these planes have assigned code points (characters), and are named.

Currently, about ten percent of the potential space is used. Furthermore, ranges of characters have been tentatively mapped out for every current and ancient writing system (script) the Unicode Consortium has been able to identify. While Unicode may eventually need to use another of the spare 11 planes for ideographic characters, other planes remain. Even if previously unknown scripts with tens of thousands of characters are discovered, the limit of 1,114,112 code points is unlikely to be reached in the near future. The Unicode Consortium has stated that the limit will never be changed.

The odd-looking code points limit (it is not a power of 2) is due to the design of UTF-16. In UTF-16 a "surrogate pair" of two 16-bit words is used to encode 220 code points in the planes 1 to 16, in addition to the use of single code unit to encode plane 0. It is not due to UTF-8, which was designed with a limit of 231 code points (32768 planes), and can encode 221 code points (32 planes) even if limited to 4 bytes.

Sometimes, the terms “astral plane” and “astral characters” are used informally to refer to the planes above the Basic Multilingual Plane (planes 1–16) and their characters.