UCS2

UCS-2, short for Universal Coded Character Set 2-byte form, is a fixed-width character encoding that uses 16-bit code units to represent Unicode code points from U+0000 to U+FFFF, i.e., the Basic Multilingual Plane. It is a predecessor of UTF-16 and is largely historical in modern Unicode implementations.

Because UCS-2 encodes only the BMP, it cannot represent characters outside that range. To encode supplementary characters such as many emoji or less common CJK ideographs beyond U+FFFF, systems using UCS-2 would fail; in UTF-16 these code points are represented with a pair of 16-bit units called surrogate pairs. Therefore UCS-2 cannot accommodate full Unicode by itself.

UCS-2 may be stored in big-endian or little-endian byte order, often referred to as UCS-2BE and UCS-2LE, respectively. The concept of endianness is similar to UTF-16, and the same byte-order distinctions apply when storing the data.

Historically, UCS-2 appeared in early Unicode and ISO/IEC 10646 implementations and in some legacy software stacks. In modern standards, UCS-2 is considered deprecated in favor of UTF-16, which preserves all BMP code points in one unit and uses surrogate pairs for supplementary planes. Most contemporary systems use UTF-8, UTF-16, or UTF-32 as standard encodings, with UCS-2 retained primarily for documentation or legacy compatibility.