The centre of the façades is usually interrupted by an entrance doorway forming a "trilithon", a pair of orthostats surmounted by a massive lintel slab. Further trilithons form a passage, which is always paved in stone. This in turn opens onto an open space, which then gives way to the next element, a pair of D-shaped chambers, usually referred to as 'apses', opening on both sides of the passage. The space between the apses' walls and the external boundary wall is usually filled with loose stones and earth, sometimes containing cultural debris including pottery shards.