Just an alias. I figure I could use a single letter command for something I do like 1/3 of the time on the commandline, especially since it's on the opposite hand from the enter key.

ls is actually pretty well optimized for an QWERTY layout already so it's not a big win, but it's something I've been doing for 25 years now so the habit is well ingrained.

Usually the alias is now:

  alias d="ls -F --color=auto" 

on Linux systems.