The rule is used to find the middle of an edge or surface by placing it across the piece so that the distances from the edges of the piece to corresponding inch, or fractional marks shall be the same, [Fig. 3], the middle of the piece being at a point midway between the marks selected.
2. The Try-square.
—The try-square may be made entirely of iron or steel or it may have a head of wood called the beam and a blade of steel. The blade is graduated into inches and fractions of an inch. As all try-squares are liable to be injured by rough usage, care should be taken not to let them drop on the bench or floor, nor should they ever be used for prying or pounding. [Fig. 4].
Fig. 4.
The try-square is used for three purposes: First, to act as a guide for the pencil or knife point in laying out lines across the grain at right angles to an edge or surface; second, to test an edge or end to see whether it is square to an adjoining surface or edge; third, to test a piece of work to see whether it is of the same width or thickness thruout its entire length.
[Fig. 5] shows the various positions assumed in lining across a piece. The beam should be held firmly against either the face side or the face edge.
The face side of a piece is the broad surface which is first made true. The face edge is the first edge which is made square to the face side and straight. These two surfaces are usually marked in some way so that they may be distinguished from the other surfaces. Their use is fully explained in [Chapter III].
Fig. 5.