straightedge

The first requisite for this is a straightedge of iron or brass, as true as possible.

It is necessary also that the straightedge be applied frequently.

The cutter is guided along the straightedge as shown in Fig. 1.

The straightedge and the compasses are the only two drawing instruments recognized in elementary geometry.

In other words, the trisection of any angle, by the use of the straightedge and compasses alone, is impossible.

This, like Postulate 1, requires the use of a straightedge for drawing the physical figure.