Comparative studies have shown that many ritual practices go back to the period when the Indians and Persians were still one people. Thus the sacrifice was even then the centre of a developed ceremonial, and was tended by a priestly class. Many terms of the Vedic ritual already existed then, especially soma, which was pressed, purified through a sieve, mixed with milk, and offered as the main libation. Investiture with a sacred cord was, as we have seen, also known, and was in its turn based on the still older ceremony of the initiation of youths on entering manhood. The offering of gifts to the gods in fire is Indo-European, as is shown by the agreement of the Greeks, Romans, and Indians. Indo-European also is that part of the marriage ritual in which the newly wedded couple walk round the nuptial fire, the bridegroom presenting a burnt offering and the bride an offering of grain; for among the Romans also the young pair walked round the altar from left to right before offering bread (far) in the fire. Indo-European, too, must be the practice of scattering rice or grain (as a symbol of fertility) over the bride and bridegroom, as prescribed in the Sutras; for it is widely diffused among peoples who cannot have borrowed it. Still older is the Indian ceremony of producing the sacrificial fire by the friction of two pieces of wood. Similarly the practice in the construction of the Indian fire-altar of walling up in the lowest layer of bricks the heads of five different victims, including that of a man, goes back to an ancient belief that a building can only be firmly erected when a man or an animal is buried with its foundations. Finally, we have as a division of the Sutras, concerned with religious practice, the Çulva Sutras. The thirtieth and last praçna of the great Kalpa Sutra of Apastamba is a treatise of this class. These are practical manuals giving the measurements necessary for the construction of the vedi, of the altars, and so forth. They show quite an advanced knowledge of geometry, and constitute the oldest Indian mathematical works. The whole body of Vedic works composed in the Sutra style, is according to the Indian traditional view, divided into six classes called Vedangas ("members of the Veda"). These are çiksha or phonetics; chhandas, or metre; vyakarana, or grammar; nirukta, or etymology; kalpa, or religious practice; and jyotisha, or astronomy. The first four were meant as aids to the correct reciting and understanding of the sacred texts; the last two deal with religious rites or duties, and their proper seasons. They all have their origin in the exigencies of religion, and the last four furnish the beginnings or (in one case) the full development of five branches of science that flourished in the post-Vedic period. In the fourth and sixth group the name of the class has been applied to designate a particular work representing it.