Emacs can order completions by column!

Emacs completions are, by default, printed in left to right, then top to bottom. This makes visually scanning large lists painful.

Fortunately, it is possible to order by column first, thus making visual scanning much easier. Here's how:

(setq completions-format 'vertical)

So instead of:

bar1 bar2 bar3
bar4 foo1 foo2
foo3 foo4 ...

You'll get:

bar1 bar4 foo3
bar2 foo1 foo4
bar3 foo2 ...