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UTF-16 is a 16-bit Unicode Transformation Format, a character encoding form that provides a way to represent a series of abstract characters from Unicode and ISO/IEC 10646 as a series of 16-bit words suitable for storage or transmission via data networks. UTF-16 is officially defined in Annex Q of ISO/IEC 10646-1. It is also described in The Unicode Standard version 3.0 and higher, as well as in the IETF's RFC 2781.

UTF-16 represents a character that has been assigned within the lower 65536 code points of Unicode or ISO/IEC 10646 as a single code value equivalent to the character's code point: 0 for 0, hexadecimal FFFD for FFFD, for example.

UTF-16 represents a character above hexadecimal FFFF as a surrogate pair of code values from the range D800-DFFF. For example, the character at code point hexadecimal 10000 becomes the code value sequence D800 DC00, and the character at hexadecimal 10FFFD, the upper limit of Unicode, becomes the code value sequence DBFF DFFD. Unicode and ISO/IEC 10646 do not assign characters to any of the code points in the D800-DFFF range, so an individual code value from a surrogate pair does not ever represent a character.

These code values are then serialized as 16-bit words, one word per code value. Because the Endianness of these words varies according to the computer architecture, UTF-16 mandates that the byte order must be declared by prepending a Byte Order Mark before the first serialized character. This BOM is the encoded version of the Zero-Width No-Break Space character, Unicode number FEFF in hex, manifesting as the byte sequence FE FF for big-endian, or FF FE for little-endian. A BOM at the beginning of UTF-16 encoded data is considered to be a signature separate from the text itself; it is for the benefit of the decoder.

The UTF-16LE and UTF-16BE encodings are identical to UTF-16, but rather than using a BOM, the byte order is implicit in the name of the encoding (LE for little-endian, BE for big-endian). A BOM at the beginning of UTF-16LE or UTF-16BE encoded data is not considered to be a BOM; it is part of the text itself.

The IANA has approved UTF-16, UTF-16BE, and UTF-16LE for use on the Internet, by those exact names (case insensitively). The aliases UTF_16 or UTF16 may be meaningful in some programming languages or software applications, but they are not standard names.

UTF-16 examples

code pointcharacterUTF-16 code value(s)glyph*
122 (hex 7A)small Z (Latin)007Az
27700 (hex 6C34)water (Chinese)6C34
119070 (hex 1D11E)musical G clefD834 DD1E𝄞

"水z𝄞" (water, z, G clef), UTF-16 encoded
labeled encodingbyte orderbyte sequence
UTF-16LElittle-endian34 6C, 7A 00, 34 D8 1E DD
UTF-16BEbig-endian6C 34, 00 7A, D8 34 DD 1E
UTF-16little-endian, with BOMFF FE, 34 6C, 7A 00, 34 D8 1E DD
UTF-16big-endian, with BOMFE FF, 6C 34, 00 7A, D8 34 DD 1E

* Appropriate font and software are required to see the correct glyphs.

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