Which letters are not used in doubles?

This is a bit of a trick question! There's no rule about which letters are *not* used in doubles.

Here's why:

* "Doubles" can refer to many things: Do you mean doubles in a game of Scrabble? Doubles in a tennis match? Doubles in math? Each of these has different rules.

* Any letter can be doubled: In Scrabble, you can double any letter using a double letter tile. In other contexts, you can simply repeat any letter to make a "double" (like "tall" becomes "taall").

So, to answer your question accurately, we need more context! Could you clarify what you mean by "doubles"?