For convenience it is hinged so as to fold.

A rule is called "two-fold" when it is made of two pieces, "four-fold" when made of four pieces, etc.

When measuring or marking from it, it can be used more accurately by turning it on edge, so that the lines of the graduations may come directly against the work.

The one in most common use in school shops, is a two-foot, two-fold rule.

Some instructors prefer to have pupils use a four-fold rule, because that is the form commonly used in the woodworking trades.