How to get emacs to list suggestions like ls (along columns)

Question

How can I get emacs to list suggestions along columns instead of rows?

In short, emacs gives me this:

In this buffer, type RET to select the completion near point.

Possible completions are:
abc1                  abc2                  abc3
abc4                  abc5                  abc6
abc7                  abc8                  abc9
bc1                   bc2                   bc3

But I would like it like this, so I can quickly scan a column:

In this buffer, type RET to select the completion near point.

Possible completions are:
abc1                  abc5                  abc9
abc2                  abc6                  bc1
abc3                  abc7                  bc2
abc4                  abc8                  bc3

Answer 1

Adding the following to your .emacs should do the trick.

(setq completions-format 'vertical)

Note: This option was introduced in Emacs 23.2.