The vajra gives its name to the whole Vajrayana vehicle, and its history is a clean example of a religion taking an object and reversing what it means.
From thunderbolt to the indestructible
In earlier Indian religion the vajra is the thunderbolt weapon of Indra, a thing that destroys. Buddhism took the object and reassigned it, so that in Vajrayana ritual the short metal sceptre, held in the right hand and paired with the bell in the left, stands for method, compassion and the indestructible, a thing that cannot be destroyed rather than one that destroys. The prong count is not decorative: five-pronged and nine-pronged forms carry different meanings, and a crossed double vajra marks the immovable base of a mandala.
The pair
The vajra is not used alone in ritual but with the bell, the ghanta, held in the left hand; the vajra is method and compassion, the bell wisdom and emptiness, and holding both at once states that the two are inseparable. The handle of the bell is itself a half-vajra, so the pair is in a sense one object divided, which makes the physical implements a compact statement of the doctrine they serve.