A single-byte character string is a character string that consists of one of the following: No character (called a null string) One or more single-byte characters supplementary characters The first version of Unicode was a 16-bit, fixed-width encoding that used two bytes to encode each character. This allowed 65,536 characters to be represented. However, more characters need to be supported because of the large number of Asian ideograms. Unicode 3.1 defines supplementary characters to meet this need. It uses two 16-bit code units (also known as surrogate pairs) to represent a single character. This allows an additional 1,048,576 characters to be defined. The Unicode 3.1 standard added the first group of 44,944 supplementary characters.