|Inventor||Laroy S. Starrett (1877)|
A combination square is a multi-purpose measuring and marking tool used in metalworking, woodworking, and stonemasonry. It is composed of a rule and one or more interchangeable heads that can be attached to the rule. Other names for the tool include adjustable square, combo square, and sliding square.
The most common head is the standard head, which is used as a square for marking and testing 90° and 45° angles. The other common types of head are the protractor head, and the centre finder head.
Combination square rules are made of steel and can be purchased with gradations in metric, imperial, or both metric and imperial. Both faces of the rule have markings, providing four different sets of markings. This allows different sides to have different graduations (eg. 1mm or 0.5mm markings) or units (ie. metric and imperial). The rule typically comes in lengths between 150mm and 600mm or between 4 inches and 24 inches.
The heads, occasionally called anvils, are attached to the rule by sliding the rule into a slot in the side of the head. The head is then tightened in place via a lock bolt or lock nut which engages with a channel running the full length of the rule, allowing the head to be tightened on at any point along the rule.
The standard or square head has three adjacent flat faces, two of them meet square to one another, and the third face is angled away at 45°. When attached one face is parallel to the rule, one face is perpendicular, and one face is at 45°. The standard head usually incorporates a small spirit level and a small removable scriber.
The protractor head has a flat reference edge which is attached to an adjustable 180° protractor or (sometimes called a turret) with a graduated scale in both directions for reading both the angle or the complement angle. The protractor head sometimes includes a small spirit level.
The centre finder head has two faces meeting at 90°, when attached one edge of the rule bisects the two faces at 45°.
The heads are manufactured from either forged steel, cast iron, die-cast aluminium, die-cast zinc, or plastic. Aluminium and zinc heads are cheaper than steel and iron, but less durable and more prone to inaccuracy. Cast iron and steel heads are also notably heavier. The heads are usually painted except for the flat machined reference faces.
As well as being used as a regular standalone rule or straightedge, the rule is used in combination with the different heads.
The standard head can be used as a:
The protractor head can be used for:
The centre finder head can be used for:
Though some earlier 19th century tools were called combination squares, the modern combination square was invented in the late 1870s by American inventor Laroy S. Starrett, and patented in 1879.[note 1] In 1880 he founded the L. S. Starrett Company in Athol, Massachusetts, United States. The tool was originally designed for machinists, but over time became commonly used in other trades, such as woodworking.