Board Thread:Questions and Answers/@comment-10116893-20130528070747/@comment-7159135-20130529002741

Okay, I see what you're getting at.

A back up strip is what is generally referred to as the second (or in some cases third or fourth) story in a comic, following the main story. So when it says Classic X-Men back up, it means read the second story in the comic.

Some of the Classic X-Men reprints have alterations in them from the original stories. For example, Apocalypse is inserted in a flashback at one point. I think this is what is meant by "Front Up" but I didn't write that list.

At any rate, I doubt if you accidently read the Classic X-Men stories in the wrong spot it wouldn't greatly reduce your enjoyment of the stories. I'm sure some fan has written a FAQ on Classic X-Men and posted it somewhere one this interwebs thing.