Derek Broughton wrote:

rfc1855 says _nothing_ about bottom posting. The word "bottom" doesn't even
appear. Of course, the RFC itself says nothing directly about top posting
either, though it does say "be sure you summarize the original at the top
of the message, or include just enough text of the original to give a
context".

"be sure you summarize the original at the top of the message" is the
essence of bottom posting.