The Vajra (Thunder Bolt)

Vajras may have nine, five or three spokes.

The spokes of a peaceful vajra meet at the tip whereas those of a wrathful vajra are slightly splayed at the end.

When paired with a bell their length can vary from four finger-widths to twenty-eight finger widths.

The upper sets of spokes of a five spoked vajra symbolize the five wisdoms, which are:

    The mirror like wisdom - that which reflects all sense perceptions is purified when one attains enlightenment and becomes the mirror like wisdom.

    The wisdom of equality - arises after all the feelings of pleasantness, unpleasantness and indifference have been purified.

    The wisdom of individual analysis - arises when the factor of discrimination, which distinguishes one object from another is purified. It enables one to benefit each sentient being according to his or her needs and disposition.

    The wisdom of accomplishing activity - arises when the basic ability to perform acts according to particular circumstances is purified.

    The wisdom of the sphere of reality - arises when consciousness is purified and becomes the mind that is the seed of the wisdom truth body of a Buddha. The five lower spokes symbolize the five mothers.

In the case of a nine spoked vajra, the upper spokes symbolize the Buddhas of the five families and the four mothers, one between each of the Buddhas. The lower spokes represent the five wisdoms and the four immeasurable wishes of love, compassion, equanimity and joy.

The sea-monster's mouth from which each spoke emerges represents freedom from cyclic existence. Of the two lotuses at the hub of the vajra, the upper one symbolizes the eight Bodhisattvas, and the lower one, the eight goddesses.

Paired with a bell the vajra represents method or compassion.

Source

http://www.buddhanet.net/e-learning/history/bud_vajra.htm