The combination square is one of those essential tools that no woodworker should be without. Even if you are just starting out as a woodworker, it won't take long to learn how valuable this simple tool can be -- it is at once a machinist’s square, a straightedge, an adjustable try square, a miter square, a marking gauge, a depth gauge, and a ruler. You can use it to set up shop machines, to true workpieces, to lay out joinery, and to fine-tune it when you are nearly done.

From Fine Woodworking #197