List all possible 2 letter combinations of 8 letters

Hi~ I'm trying to write a script that will list all 2 letter combinations of given number of letters. For example, let's say I entered 8 letters "a, b, c, d, e, f, g, h". I want the script to spit out all 2 letter combinations-- i.e. ab, ac, ad, ae, etc.... Also, the script should make sure it doesn't repeat any combinations-- I really have no idea where to start on this one-- the only thing I know is that it will most likely be within some sort of for/while loop... can someone help? Thanks, -Ben