Several years we did a popular skill builder on the speed square, a triangular measuring tool created for roofing carpenters that has been more widely adopted as a convenient and versatile measuring and marking tool. Perhaps even more universally useful is the combination square, a sliding ruler and angle-measuring (45/90) tool (which frequently includes a level).

This video does an excellent job of running through many of the major uses and functions of the combination square. While they are particularly talking about working with wood and woodworking machinery, a combo square can be used to measure and mark all sorts of work materials and to test and calibrate any type of machine that has adjustable-angle work surfaces.

Here are some of the things a combination square can do (as outlined in the above video):

  • Marking straight lines perpendicular to a factory or true edge of a workpiece.
  • Drawing parallel lines along a workpiece.
  • Laying out an accurate rectangle on a workpiece.
  • Marking all sides of a post or beam.
  • Finding the center of a circle or the center of a pipe or dowel.
  • Testing tools and workpieces for true.
  • Calibrating tools to make them true.
  • Setting table saw blades.
  • Setting the angles on a miter gauge.
  • Measuring router bit depth (and other depths such as dado cuts).

Here’s another, similar video, from Beauty and the Bolt, which also runs through the basics of properly wielding a combination square:

But what if the square ain’t square? Cheap combo squares are often not true (or don’t remain so). In this video, John Heisz shows you how to test your square for square and how to adjust it to make it true: