There are no definite dimensions for a straight-edge. Any piece of wood that is straight and convenient to use can be so called; the size and the length depending on the work for which it is to be used, from a common ruler to a long board. The edge of a large carpenter's square is handy for short work. Clear white pine or straight-grained mahogany is good for straight-edges, but a straight-edge is not the easiest thing for a beginner to make, and you will do well to find something straight to use for a while until you acquire the skill to make one - or get the carpenter to make you one, which he will do for a very small sum or for nothing.
To test a straight-edge, mark a line by it, then turn the straightedge over and see if it still coincides with the line, or mark another line and see if it coincides with the first one. Try your straight-edges by this test once in a while, as they are liable to become crooked. In turning the edge over, however, do not reverse the ends, as in case of an undulating curvature the curves may agree and give you the impression that the edge is straight when it is not. In the first case shown in Fig. 684 (exaggerated) this would not happen, but in the second case (also exaggerated) it might. See Marking.