It is said that with the rise of Tantric practice in all three major religions of India Brahmanism, Jainism and Buddhism, mantras and yantras are became very prominent.
Tantra is the realization of profound knowledge with the aid of mystic diagrams (yantras) and words possessing esoteric meanings (mantras). The Tantric cult of all three religions of India lays special stress upon the mantras, i.e. prayers and formulae, bija, i.e. syllables of mystic significance peculiar to each deity, yantra, i.e., diagrams drawn on paper or inscribed in precious stone metal etc. mudras, i.e. special position of fingers, and nyasas, i.e. placing the deities on the different parts of the body by touching them with finger-tips and the palm, mostly of the right hand. These are the means by which the sadhaka invokes, and identifies himself with his chosen deity (ishtadevata).
As mentioned above, Brahmanism, Jainism and Buddhism in course of time developed and incorporated Tantra and started making use of mantras and yantras. Mantras, in the practices, are located in the body by nyasa they are also placed outside in a yantra (diagram) which represents, as it were, the body of the deity. The Vedic gods were invoked without any image and on the sacred grace (barhis, kusha) in a sacrificial field, and the sculptured deities that followed were seat on lotuses. A Yantra has, like the sacrificial field, gates or doors (dvara) and a painted or inscribed or incised seat; and all together or in the various petals of the lotus, inscribed or drawn, or in the different parts of other types of diagrams are inserted the latter of the bijamantra of the deity worshipped. Coloured rice-powder is sometimes used to fill the geometrical figures created by the drawing, just as five differently coloured substances are used to cover the ground of the mandala. As drawings and painting are ephemeral, in course of time more permanent yantras were made. For this purpose some metals like gold, silver and copper, or an alloy of the three metals (trilauha) or some precious stone like crystal, emerald, ruby, coral etc were used, as per the selection of the sadhaka. A strict instruction has always been given pertaining to keep the yantra properly and carefully. Care must be taken that the yantra does not get mutilated, faded out by use, cracked, burnt or broken in which case it would have to be immersed into a sacred stream or place of pilgrimage or the ocean.
Every deity has his or her own Yantra, but the most famous of these Yantras is the present one, i.e. Shri-Yantra; a lavish praise has been bestowed on this Yantra. In its various constituent parts it is supposed to represent the origin, maintenance and dissolution of the world of things, the dot in the middle representing the unitary world-ground. The bija-mantra of the Shakti goddess is mentally placed in the various projections of its constituent triangles and in its circles and squares.
It is believed that by keeping the Shri yantra one can bring peace, prosperity and ecological friendly environment in the house. The yantra should be installed on a sacred place after the performance of proper rituals under the guidance of a competent priest.
J. N. Banerjea, Pauranic and Tantric Religion, Calcutta, 1966
A. Waddell, Buddhism and Lamaism of Tibet, Delhi, 1979 (reprint)