cavetto

Within the cavetto there is a portrait of Trotsky, a red star with crossed hammer and sickle, a facsimile of Trotsky's signature, and a worker and a Red Army soldier holding a sign that reads '1917 Red Army 1922'.

cavetto: a concave molding the outline of which is a quarter circle.

Rectangular plinth with overhanging cavetto moulding (a hollow moulding with its profile being a quadrant of a circle, mainly used in cornices); a podium with cavetto and roll mouldings at the base and battered sides.

Think of names like cavetto, ovolo, scotia and torus - and those are just the simple shapes.