The checksum letter is calculated by converting the letters into numbers, "i.e.", where A=1 and Z=26, potentially giving seven individual numbers from each registration plate. However, only two letters of the prefix are used in the checksum. For a three-letter prefix, only the last two letters are used; for a two-letter prefix, both letters are used; for a single letter prefix, the single letter corresponds to the second position, with the first position as 0. For numerals less than four digits, additional zeroes are added in front as placeholders, for example "1" is "0001". SBS 3229 would therefore give 2, 19, 3, 2, 2 and 9 (note that "S" is discarded); E 12 would give 0, 5, 0, 0, 1 and 2. SS 108 would be given as 19, 19, 0, 1, 0, 8.