A triumphal arch is a massive freestanding archway, with a rounded arch inscribed in a rectangular structure. The thickness of each leg is in somewhat close proportion to its width, and the surface of the structure is usually decorated with classical ornament. The top of the structure may be flat or can hold a statue or some elaborate decoration. There may or may not be side arches flanking the central one.

The opening sits within a substantial rectangular body rather than forming the whole outline of the structure. The legs have depth as well as width, producing the stated massive form.

A simpler arch frames an opening without necessarily having that rectangular body. Side arches and decoration can vary while the triumphal-arch form remains recognizable.