> (so is short for "source".)
> se nu
> [SEt NUmber - set line numbering on.]

Just in case it wasn't clear, vi does not require you to abbreviate commands.
Biff the dog will not chase you down if you actually type 'source' or
'set number'.  I may type  :se sw=2 ai  if I've just sat down at a new system
and I'm in a hurry, but if I'm writing a .exrc file I will generally write
set shiftwidth=2 autoindent
instead, so I don't need my secret decoder ring if I look at it a month later.

-Chap