An isogram is a meaningful string, generally a word or a phrase, made of distinct letters. When playing with combinatorics, it’s fun to learn about the existence of such words. Let’s take the word COMBINATORICS, then there are 13!/(2!2!2!) ways to form a new 13 letters words from its letters. In fact, we must divide 13! by those letters which are repeated so that we don’t overcount. In this example, “o”, “c” and “i” appear 2 times each.
One of the longest english isogram is UNCOPYRIGHTABLES. With 16 letters, and no repeated letters, the number of possible arrangements amounts to 16!. Pretty big to picture up…
http://en.wikipedia.org/wiki/Isogram