Pros: inexpensive tool to simplify the door-hanging process
Cons: slower and more work than using a router
While it sounds as if it's a device most people consult before making that annual resolution to lose fifteen pounds, a Butt Gauge (or Butt Template) actually has nothing to do with the dreaded marital pop quiz, "Honey, does this gauge make my butt look big?" Truth be told, I'm not ever certain why anyone would call it a "gauge"; I'm more inclined to call it a template. Yes, I own one and, thanks to a certain door-destroying blonde dog, it's been used several times.
The "butt" part of the name refers to butt hinges, a pair of which (if not three) is on every door of the room you're in right now. They're those big, flat hinges that hold a door in its jamb and, in case you hadn't noticed, they're inset into both door and frame so that the surfaces are more or less flat (carpenters say they're mortised into the frame and door, but you don't need to know that). Hinges are a standard size (at least in residential doors), but the number and position varies widely from door to door. So if you're stuck replacing a door (been there, done that), you'll have to cut a mortise for each hinge so that you can mount the door in its frame. If you haven't invested in a router and hinge-mortise template, you'll be carving out the mortises by hand with a wood chisel.
Here's where a butt template comes in: it's used to mark the location of a standard 3½-inch butt hinge and it's also used to start the mortise. Once you've (c-a-r-e-f-u-l-l-y) located the hinge position, you hold the gauge in place and WHACK! it with a mallet, which causes the raised edges to cut into the wood. This gives you a nice, clean cut marking the outline of the mortise. There's also a scriber-style depth gauge you can slide along the open edge of the mortise to mark proper depth. Once that's done, it's up to you to clean out the mortise with a nice, sharp wood chisel.
This butt gauge is sized for standard residential hinges. The mortise it marks has square corners, although commercial hinges now have rounded corners because most mortises are cut with a router these days. While it's more tedious to use a chisel than a router, I find it more satisfying to use my hands than a ¾-HP motor (except when I have lots of mortises to cut...)
Mine's been used on half a dozen doors, and a chunk of the sharp edge along one side has chipped off. It still functions just fine, though - I merely use an extra vertical stroke of my chisel. I've also noticed that the gauge edges and, especially, the scriber are quite sharp and quite sneaky. If you get one, I strongly advise keeping the gauge in its original packaging or covering the sharp edges with duct tape between uses. You don't want to find this thing with an unwitting fingertip while rooting around in a toolbox.
Another indispensable door-hanging tool:
Door Lock Installation Kit