# AA and AB Rules

When the numerical value exceeds 26, a combination of letters is produced. The letterStyle specifies how letters are combined. If AA is specified, then the letters are repeated. If AB is specified, then the letters accumulate in a fashion similar to that of an odometer. Specifically, the revision number is first converted to a base-0 index, and then converted to a base-26 number.

When AA is specified, then the lowest base-26 digit is used as a 0–based index to select a letter, and that letter is repeated as many times as the number is a multiple of 26, plus 1.

When AB is specified, then the highest base-26 digit is decremented, and each base-26 digit is used as a 0–based index to select a letter. (If letters are omitted, then the base will not be 26, but the number of letters that remain in the alphabet.)

See the table for examples.

Number Value of the Number Using AA Rules Value of the Number Using AB Rules
1 A A
2 B B
26 Z Z
27 AA AA
28 BB AB
52 ZZ AZ
53 AAA BA
54 BBB BB
78 ZZZ BZ
79 AAAA CA
676 ZZZZZZZZZZZZZZZZZZZZZZZZZZ (676 is 26*26, so there are 26 Zs) YZ
677 AAAAAAAAAAAAAAAAAAAAAAAAAAA (27 As) AAA