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The moonstone (Sandakada Pahana) is a permanent feature of the buddhist building of all historical periods. This is a semi circular piece of stone which stood at the foot of a flight of steps in most buddhist buildings. Although the moonstone is generally semi circular, Sometimes you can come across square moonstones.
It is thought that the moon stones originated as blank square stone and later developed in to a semi circular shape. This again developed to include multitude of carved decorations in later stages. But the moonstones in the monasteries where the forest dwelling monks lived maintained the blank semi circular shape.