A structural form in a building that spans openings by arranging wedge-shaped blocks (voussoirs), such that the pressure exerted by the part of the building above the opening is channeled to the vertical supports of the arch on either side of the opening. There are many varients of the form of the true arch; most common is the semicircular, curved type, but also known are the stilted, segmental, skewed, three centered and flat arch. A false arch can be formed by other means such as corbelling (q.v.). The basis of traditional Greek architecture is post-and-lintel construction. Greek merchants, envoys and other travellers must have observed the use of the arch in the Near East and Egypt for millennia but apparently did not find it appropriate or necessary in their own building. It has been argued that the arch and barrel vault were not adopted for use in Greece until the engineers and masons who accompanied Alexander the Greats invasion of Asia in 334 B.C. were exposed to arcuated construction, and that they subsequently spread through Macedonian influence. Although this theory is complicated by disputed dating of some Macedonian barrel-vaulted tombs (especially the so-called Tomb of Philip II at Vergina [ancient Aegae]), the arch and arcuated construction in Greece seems to appear fully developed in the Early Hellenistic period but is rarely used before then. The Greeks used the arch and barrel vaults primarily in subterranean tombs, passageways, gateways, and reservoirs.