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). Plane 0 is the Basic Multilingual Plane (BMP), which contains most commonly used characters. The higher planes 1 through 16 are called "supplementary planes". The last code point in Unicode is the last code point in plane 16, U+10FFFF. As of Unicode version 15.0, five of the planes have assigned code points (characters), and seven are named.
This article needs additional citations for verification. (July 2016)
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 would still be able to encode 221 (2,097,152) code points (32 planes) even under the current limit of 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 327 blocks defined in Unicode 15.0 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.