UCS-2

UTF-16 (16-bit Unicode Transformation Format) is a character encoding capable of encoding all 1,112,064 possible characters in Unicode. The encoding is variable-length, as code points are encoded with one or two 16-bit code units. (also see Comparison of Unicode encodings for a comparison of UTF-8, -16 & -32)

UTF-16 developed from an earlier fixed-width 16-bit encoding known as UCS-2 (for 2-byte Universal Character Set) once it became clear that 16 bits were not sufficient for Unicode's user community.

In the late 1980s, work began on developing a uniform encoding for a "Universal Character Set" ( UCS) that would replace earlier language-specific encodings with one coordinated system. The goal was to include all required characters from most of the world's languages, as well as symbols from technical domains such as science, mathematics, and music. The original idea was to expand the typical 256-character encodings requiring 1 byte per character with an encoding using 2¹⁶ = 65,536 values requiring 2 bytes per character. Two groups worked on this in parallel, the IEEE and the Unicode Consortium, the latter representing mostly manufacturers of computing equipment. The two groups attempted to synchronize their character assignments, so that the developing encodings would be mutually compatible. The early 2-byte encoding was usually called "Unicode", but is now called "UCS-2".

Early in this process, however, it became increasingly clear that 2¹⁶ characters would not suffice, and IEEE introduced a larger 31-bit space with an encoding (UCS-4) that would require 4 bytes per character. This was resisted by the Unicode Consortium, both because 4 bytes per character wasted a lot of disk space and memory, and because some manufacturers were already heavily invested in 2-byte-per-character technology. The UTF-16 encoding scheme was developed as a compromise to resolve this impasse in version 2.0 of the Unicode standard in July 1996 and is fully specified in RFC 2781 published in 2000 by the IETF.

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