The ASCII character set defines the characters from 0 to 127 and an extended set from 128 to 255. Several alternative single-byte character sets, primarily European, define the characters from 0 to 127 identically to ASCII, but define the characters from 128 to 255 differently. With this extension, 8-bit representation is sufficient for defining the needed characters in most European-derived languages. However, some languages, such as Japanese Kanji, include many more characters than can be represented with a single byte. These languages require multibyte coding.
A multibyte character set consists of both one-byte and two-byte characters. A multibyte-character string can contain a mix of single and double-byte characters. A two-byte character has a lead byte and a trail byte. In a particular multibyte character set, the lead and trail byte values can overlap, and it is then necessary to use the byte's context to determine whether it is a lead or trail byte.