Pay no attention to the letter $Q$. $Q$ is just a "short-hand" for "the $N$th letter". Think of $Q$ as a kind of variable, rather than a letter itself.
In other words, ignore the Q bit, and keep reading. If you like, think of it as A, B, C, ..., L, and pretend that was just a typo. Keep reading.
Let me give you some advice about reading papers. If you find some detail you don't understand, often you can keep reading and see if you can still understand what is going on. Sometimes there will be a typo in the paper that is small or inconsequential. Sometimes it's a detail but if you keep reading you can still understand much of the rest of the paper. Sometimes the paper is actually correct and once you absorb the rest of the ideas in the paper, when you go back and re-read the paper you'll realize what the authors actually meant. (Of course, occasionally you'll discover a serious fundamental flaw in the paper. But don't jump to assuming that's the case; that should be your last hypothesis, rather than your first hypothesis.)
If you still can't understand the paper, try to find other papers that cover related topics -- e.g., that it cite or that cite it -- and build up your understanding of the area. Then, come back and read the paper again.
Often, if you follow these strategies, you'll be able to work out what is going on. These are useful skills -- they will make you more self-reliant and more effective at getting value out of the research literature.