Unicode defines 26 regional indicator symbols for constructing flag emoji.
The symbols correspond to the 26 letters of the basic Latin alphabet, from REGIONAL INDICATOR SYMBOL LETTER A through Z. Two regional indicators are normally combined to represent a country or territory flag, using the same letter pair as its two-letter ISO 3166-1 alpha-2 code.
For example, the letters U and S form the United States flag sequence, while G and B form the United Kingdom sequence. The regional indicators are not ordinary alphabetic letters; they are special symbols designed for this flag mechanism.
Flag rendering is platform-dependent. A system may display a complete flag, show two letter-like symbols, or use a missing-glyph design if it does not support the sequence. Not every two-letter combination corresponds to an officially supported flag.