completions-format

completions-format is a variable defined in `minibuffer.el'.
Its value is

horizontal

Documentation:
Define the appearance and sorting of completions.
If the value is `vertical', display completions sorted vertically
in columns in the *Completions* buffer.
If the value is `horizontal', display completions sorted
horizontally in alphabetical order, rather than down the screen.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 23.2 of Emacs.