When adding letter suffixes to exhibits, it's common to have to go past Z, and begin a two letter sequence. AA, AB, AC, and so on. This formula will allow you to generate a list of two letter pairs running in order from A to ZZ.
This formula can be placed anywhere on a worksheet, so long as there is a helper column next to it with a list of consecutive numbers from 0 to 701. In this example the formula points to G9 in which I have entered zero. Pulling down the formula with CTRL + D to be adjacent to the list of numbers, will generate a list of letters from A to Z, then from AA, AB, AC . . . to AZ, then from BA to BZ, and from CA all the way to ZZ.
The formula first checks to see if the first 26 letters of the alphabet have already been generated, and then generates the two letter suffixes.
Thanks to Sundar for the idea first posted here.