Yr Allor are thought by many to be the remains of a three stone cove, along the lines of larger examples at Avebury and Stanton Drew (the Glandy Cross complex is often compared to Avebury because of the density of sites that once stood here). There are a number of accounts describing the third stone, and the existance of a third stone tends to be accepted by most people, but the site may not have been a cove. In his excellent book Prehistoric Preseli N.P. Figgis writes...
Investigations have shown that the leaning stone was deliberately propped that way, which undermines the idea that Yr Allor might have been a cove...According to Figgis an area of cobbling and some early Bronze Age pits were found around the two stones, and nearby a number of stone flakes were found, suggesting that Neolithic axe making had been taking place.
The name Yr Allor means the altar in Welsh.
Though according to Aubrey Burl in Prehistoric Avebury, the third stone lies nearby in a ditch. He also, countering Figgis, says that it is more likely to be a cove, a) because the two stones lie near to the circle-henge of Meini- gwyr, and b) because neighbouring tombs have box-like chambers.