In the Vedic period three well-defined literary strata are to be distinguished. The first is that of the four Vedas, the outcome of a creative and poetic age, in which hymns and prayers were composed chiefly to accompany the pressing and offering of the Soma juice or the oblation of melted butter (ghrita) to the gods. The four Vedas are "collections," called samhita, of hymns and prayers made for different ritual purposes. They are of varying age and significance. By far the most important as well as the oldest--for it is the very foundation of all Vedic literature--is the Rigveda, the "Veda of verses" (from rich, "a laudatory stanza"), consisting entirely of lyrics, mainly in praise of different gods. It may, therefore, be described as the book of hymns or psalms. The Sama-veda has practically no independent value, for it consists entirely of stanzas (excepting only 75) taken from the Rigveda and arranged solely with reference to their place in the Soma sacrifice. Being meant to be sung to certain fixed melodies, it may be called the book of chants (saman). The Yajur-veda differs in one essential respect from the Sama-veda, It consists not only of stanzas (rich), mostly borrowed from the Rigveda, but also of original prose formulas. It resembles the Sama-veda, however, in having its contents arranged in the order in which it was actually employed in various sacrifices. It is, therefore, a book of sacrificial prayers (yajus). The matter of this Veda has been handed down in two forms. In the one, the sacrificial formulas only are given; in the other, these are to a certain extent intermingled with their explanations. These three Vedas alone were at first recognised as canonical scriptures, being in the next stage of Vedic literature comprehensively spoken of as "the threefold knowledge" (trayi vidya).