Euclid’s insistence (c. 300 bc) on using only unmarked straightedge and compass for geometric constructions did not inhibit the imagination of his successors. Archimedes (c. 285–212/211 bc) made use of neusis (the sliding and maneuvering of a measured length, or marked straightedge) to solve one of the great problems of ancient geometry: constructing an angle that is one-third the size of a given angle.

Given ∠AOB, draw the circle with centre at O through the points A and B. Thus, OA and OB are radii of the circle and OA = OB.

Extend the ray AO indefinitely.

Now take a straightedge marked with the length of the circle’s radius and maneuver it (this is the neusis) into position to draw a line segment from B through a point C on the circle to a point D on the ray AO such that CD is equal to the circle’s radius; that is, CD = OC = OB = OA.

∠AOB = ∠ODC + ∠OBC, because ∠AOB is an angle external to ΔDOB and an external angle equals the sum of the opposite interior angles (∠AOB + ∠BOD = 180° = ∠BOD + ∠ODB + ∠DBO).

∠OBC = ∠OCB (by step 4) = ∠ODC + ∠COD (by step 5) = 2∠ODC (by step 4).