Bill Houghton, Sebastopol, CA
For a long time, I didn't know why people would buy double squares instead of combination squares. After all, a combination square gave you a 45 degree miter square, too. But then I acquired two (4" and 6") at yard sales, and now I know.
First, the stock (handle) on a double square, at least in the 6" size, tends to be longer than that on a combo square, giving you more registration against the edge.
Second, I often find myself, in the sorts of work I do, marking off holes a set distance apart at consistent short distances from the edge, for driving screws to, say, hold a panel in place (we're not talking foine woodwork here). With a double square, I can set the beam (rule) to the distance-from-edge and use a long rule or tape measure to make tick marks at the distances along the edge, from screw to screw. With the double square, I can then strike a line at the tick mark to bring it to the edge, flip the square around, and mark the distance from the edge; and I've got crosshairs showing me where to drill the holes for screws.
Now that I own them, I wouldn't be without them.