Invented in the late 19th century, the combination square is one of those tools that modern technology really can’t improve upon. In its basic form, the combo square consists of a 12" metal ruler and a sliding, triangular head with two flat surfaces—one that measures 45 degrees; the other, 90 degrees. Squares can be used in metal-work and stone masonry, but typically they’re for woodworking projects that require you to measure and mark straight lines and angles—so, pretty much any wood-working project. Using an adjustable nut, you can loosen and slide the ruler to one end of the head to measure 90 degrees or to the other end to measure 45 degrees. If you have a narrow enough board you can position the handle somewhere in the middle of the ruler to measure both angles on either side.
Combination squares also come with a liquid vial in the center of the handle, which you can use to test whether a surface is level (if horizontal) or plumb (if vertical). The combo square has a scary geometry-class look to it, but don’t be intimidated.
Here’s how it works: To draw a straight line on a board, place your pencil where you want the line and bring the edge of the ruler to meet the pencil while laying the 90-degree angle side of the handle parallel against the edge of the board. Holding the pencil against a special pencil notch on the ruler blade, move the square and pencil up together to draw the line. Some models come with a scratch awl that can be used for marking lines and angles in lieu of a pencil.—Katherine Ludwig