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This tool is essentially an adjustable hand square, with a couple of clever advantages.
It consists of a rigid steel rule, usually twelve inches long (though sometimes combination squares have rules up to twenty-four inches in length), with a headpiece that slides along its length. A knurled nut and set screw are used to fix the headpiece to the rule at any point along its length, depending upon the purpose to be served.
The headpiece has both a ninety-degree edge and one that forms a 45-degree angle with respect to the rule. The 45-degree angled edge accounts for one of the tool’s alternate names, the 45-degree miter square. It’s ideal for marking (and checking) both ninety-degree crosscuts and miter cuts.
The purposes vary: The combination square can be used as a try square, to determine the squareness of a piece of joint; like the speed square, it can also be used as a saw guide. When the head is set at the end of the rule, the combination square can measure heights. It can also be adjusted to measure depths, and some people find it’s handiest for marking.
There is a spirit (bubble) level in its handle, so the combination square can be used for leveling. Some models even have a scribe in the handle.
Not all combination squares are created equal. They range in cost from about $15.00 to as much as ten times that price. The most expensive models come equipped with two additional parts: a protractor head, for marking and measuring angles, and a center head, for locating the center of a circular or cylindrical workpiece. But it isn’t the added elements that account for the higher price.
The explanation is that the best combination squares are precision tools, useful for accurate work requiring tolerances beyond those needed by most woodworkers. However, if your work involves pattern-making or machine make-readies, for example, a more sophisticated combination square may prove to be a wise (even lifelong) investment. A top-quality square will stand up to lots of abuse without losing accuracy.