A tetrapylon (plural tetrapyla; Greek: τετράπυλον, lit. 'four gates'; Latin: quadrifrons, lit. 'four fronts', also used in English) is a rectangular form of monument with arched passages in two directions, at right angles, generally built on a crossroads. They appear in ancient Roman architecture, usually as a form of the Roman triumphal arch at significant crossroads or geographical "focal points".
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A tetrapylon was effectively a 'doubling' of the original triumphal arch form; with a total of four major arched openings, one on each side of the structure (one pair of openings opposite each other along one axis, and a second pair of openings of equal or lesser prominence perpendicular to the first pair; hence a structure with two barrel vaulted passageways, in the form of a cross). Roman examples are usually roughly square in plan, with the crossing archways of the same size; in some later examples, the plan is oblong, with the longer sides having a larger archway as for example at the Arc de Triomphe in Paris.
The three-arched form is called an octopylon as it has eight piers. As it still has four faces it can still be called a quadrifrons. The tetrakionion is another variant, with four groups of columns, usually four in each, that were never connected by stone.[1]
Called chahartaq, similar structures were built in Sasanian architecture and perhaps earlier as well as later periods in the architecture of Iran, some apparently as parts of the fire temples of Zoroastrianism. These normally had small domes above, which Roman examples did not. Since many Roman tetrapylons are in Eastern parts of the empire once ruled by the Persians, influence has been proposed, although Roman examples follow the style of Greco-Roman classical architecture.[2]