Greek temples are often categorized in terms of their ground plan and the way in which the columns are arranged. A prostyle temple is a temple that has columns only at the front, while an amphiprostyle temple has columns at the front and the rear. Temples with a peripteral arrangement (from the Greek πτερον (pteron) meaning "wing) have a single line of columns arranged all around the exterior of the temple building. Dipteral temples simply have a double row of columns surrounding the building. One of the more unusual plans is the tholos, a temple with a circular ground plan; famous examples are attested at the sanctuary of Apollo in Delphi and the sanctuary of Asclepius at Epidauros.