A basic framing square is L-shaped and looks like a metal ruler bent at a 90-degree angle. That corner allows you to join and double-check pieces to be sure they are truly square. A better choice for most jobs, though, is the speed square, which is shaped like a right triangle. Its 90-degree corner lets you check for squareness, but the side opposite that right angle is marked like a protractor, with degrees that help you cut all kinds of angles, whether for mitered corners and other custom cuts.
Another advantage of the speed square is the flat edge along one side that lets you butt it up against one side of a job or position it securely along the edge of your work piece. You can even use it as a cutting guide when you’re using a circular saw.
For more complicated jobs, you may want to consider an adjustable, T-shaped combination square. The trunk of the T can be slid along the header and tightened at any point, making it extremely useful for any number of jobs.