What is the counterpart of an appendix?

Solution 1:

When describing parts of a book, the "preface" or "prologue" are common.

Solution 2:

Was expanding my comment to @ptomato's answer but didn't get it committed within the 5-minute mark, so am elevating this to a full answer.

In books, "foreword" (not to be confused with "forward") and "introduction" are also common. However:

In "appendix", the "ap-" seems to just convey "on" or "with"; it's the "pend" that conveys "hanging", thus "downward" or "following". So really the opposite of "append" might be something like "appose" or "adhere". "Prepend", notwithstanding its documented use, seems coined from a complete misunderstanding.