I carry what I want to carry.
But if you must know, I carry a Glock 21 with three standard capacity magazines (for a total of 40 rounds) everywhere I go. This would indeed be a backup gun and would go on my ankle since my belt is pretty full most of the time. It would share duties with a Smith & Wesson 642.
And yes, it would be a backup gun, but if I want to carry it as a primary, I can and will.