The appendix is what is appended to something. What do we call something that is prepended to something else?
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.
Accordingly to http://encyclopedia2.thefreedictionary.com/prepend there is no such thing as "to prepend"
The correct English word is "prefix;" for example, "the header is prefixed to the packet." See prefix.
prefix: The beginning or to add to the beginning