The only time I have a defined "back-up" is in a police uniform, when my primary duty pistol is the most likely weapon I will deploy if things go bad. But, even the duty pistol can be temporarily relegated if I deploy my shotgun or rifle, and in a foot chase over broken terrain, I may pull my SP101 .357 snubby from its ankle rig, to make running and jumping a bit easier. "Back-up" is a relative term. On my own time, out of uniform, the snubby may be in a belt holster, while an equal or larger weapon is in a pocket or off-body carry. Either weapon may then become the "go-to" gun, depending on whether I see trouble brewing, and perhaps stand or walk with my hand on the pocketed snubby, or discreetly pull the service-sized handgun from its off-body carry, rather than do a conventional and very obvious draw from the hip holster. My normal minimum firearm while in public is an SP101 snubby, I have three of them, and usually carry two handguns on my own time. Whether this continues after I retire remains to be seen; my comfort level may change.