Generally speaking, a combination square comes with a standard head that can be used for measuring 90 degree and 45 degree angles. You can also purchase protractor heads to measure any angle between the rule (or blade) and the head, as well as a third type of head that will locate the center of a circle. The length of most combination square rules is 12″, though longer rules can be purchased.
In addition to checking and marking basic angles, the position of the standard head can be adjusted by loosening the adjustable knob and sliding the rule so a mark can be made a certain distance away from the edge of a workpiece. This function can also be used as a depth gauge to check the depth of a hole or other surface and for transferring dimensions.
A small spirit level is included in most standard heads. A small, sharp scribe is housed in a hole in the standard head. It can be used to scribe marks or mark hole locations. The rule can also be used to check for flatness on workpieces. When considering a purchase make sure the markings on the rule are clear to read.
Price Range: $10 – $140 (more for additional heads)
Types of HeadsCutters: Standard, Protractor, Centre Finding
Common Rule Lengths: Typically 12″, but available in 4″, 6″, 18″ and 24″
Like any tool, knowing what they can and can’t do is critical. The interesting thing about a combination square is they can do so many things.
Dropping a combination square is very likely going to throw its accuracy off. Not only might the head not sit properly on the ruler, but the ruler might get dented or bent, causing inaccurate readings.
Reference the standard head of a perfectly straight edge and mark a line. Flip the square over and mark another line immediately beside the first. If the lines are parallel the square is accurate.
A square is only going to be as good as the workpiece is square and true. There are many reasons to dress stock straight and true; providing accurate layout tools the ability to reference off a straight edge is one of them.
Medium and large squares have their purpose, but I bet the majority of your usage will be met with a small square. They’re also easier to carry in an apron so you can always have it on-hand.
Rob is a studio furniture maker and the editor at Canadian Woodworking & Home Improvement.
