Hand squares have been used for most of recorded history, and for good reason.
In its various forms, this tool is also known as a sliding bevel, angle bevel, bevel square, sliding T-bevel, or adjustable try square.
The carpenter's square is a deceptive tool. It looks simple?it's a ruler with an elbow, a straightedge that turns a corner. Yet it is a great deal more, because it can function as a sophisticated arithmetical guide to many of the complexities of laying out a wood-frame house.