Same as 1..200, really ;)

Basically you just use letters instead of numbers.

a..z yields the 26 characters of the alphabet,
a..az yields those and another 26 prefixed with a
a..bz yields those and another 26 prefixed with b
and so on. Same system, more "numbers" ...

Incidentally a..gr yields 7*26 + 18 "numbers".