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[a] The encoding is variable-length as code points are encoded with one or two 16-bit code units. UTF-16 arose from an earlier obsolete fixed-width 16-bit encoding now known as UCS-2 (for 2-byte Universal Character Set), [1] [2] once it became clear that more than 2 16 (65,536) code points were needed, [3] including most emoji and important CJK ...
A range of code points in the S (Special) Zone of the BMP remains unassigned to characters. UCS-2 disallows use of code values for these code points, but UTF-16 allows their use in pairs. Unicode also adopted UTF-16, but in Unicode terminology, the high-half zone elements become "high surrogates" and the low-half zone elements become "low ...
A numeric character reference refers to a character by its Universal Character Set/Unicode code point, and a character entity reference refers to a character by a predefined name. A numeric character reference uses the format &#nnnn; or &#xhhhh; where nnnn is the code point in decimal form, and hhhh is the code point in hexadecimal form.
The UCS includes 2048 code points in the Basic Multilingual Plane (BMP) for surrogate code point pairs. Together these surrogates allow any code point in the sixteen other planes to be addressed by using two surrogate code points. This provides a simple built-in method for encoding the 20.1 bit UCS within a 16 bit encoding such as UTF-16.
For U+0800 to U+FFFF, the remaining characters in the Basic Multilingual Plane and capable of representing the rest of the characters of most of the world's living languages, UTF-8 needs 24 bits to encode a character while UTF-16 needs 16 bits and UTF-32 needs 32. Code points U+010000 to U+10FFFF, which represent characters in the supplementary ...
A code point is represented by a sequence of code units. The mapping is defined by the encoding. Thus, the number of code units required to represent a code point depends on the encoding: UTF-8: code points map to a sequence of one, two, three or four code units. UTF-16: code units are twice as long as 8-bit code units.
The number of code points in each block must be a multiple of 16. A block may contain code points that are reserved, not-assigned, etc. Each character that is assigned, has a single "block name" value from the 338 names assigned as of Unicode version 16.0. Unassigned code points outside of an existing block have the default value "No_block".
For UTF-8, the BOM is optional, while it is a must for the UTF-16 and the UTF-32 encodings. (Note: UTF-16 and UTF-32 without the BOM are formally known under different names, they are different encodings, and thus needs some form of encoding declaration – see UTF-16BE, UTF-16LE, UTF-32LE and UTF-32BE.) The use of the BOM character (U+FEFF ...