The whole body of scriptures dealing with the religious ceremonial and practice of Hinduism is referred to by the term kalpa, 'usage'. It is regarded as one of the vedangas or auxiliary vedic studies. As the Upanishads embody the speculative side, and the Brahmanas the dogmatic side of the Vedas, so the kalpas form the text of the ritual side. A sacred character was never attributed to the kalpa sutras (ritual texts), because they were compiled merely to meet the practical needs of everyday life. The kalpas include the shrauta sutras and the smarta sutras.
The shrautasutra are so called because they are based on the shruti, especially the Brahmanas. The text deals with ceremonials and rules for the application of mantras and brahmanical observance in connection with vedic sacrifice. They teach the laying of the three sacred fires, the new and full moon sacrifices, and the soma sacrifices. Their operations call for a number of professional priests and hierophants.
The first ceremony of all is the setting up of the sacred fires (agni-adheya), which are kindled by the sacrificer and his wife with the firesticks, and are thereafter to be regularly maintained.the shrautarites, fourteen in number, are divided in the two main groups of seven oblation (havis) sacrifices and seven soma sacrifices. Different forms of the animal sacrifice are classed with each group. The havis sacrifices consist of offerings of milk, ghee, porridge, grain, capes. The commonest is the Agnihotra the daily morning and evening oblation of milk to the three fires. The most important of the others are the new and full moon sacrifices (darshanapurna-masa) and those offered at the beginning of the three seasons (chaturmasya). Besides some other recurrent sacrifices, there are very many which are to be offered on some particular occasion, or for the attainment of some special object.
The various kinds of soma sacrifices were much more complicated. Even the simplest and fundamental form, the Agnishtoma ("Praise of Agni") required the ministrations of sixteen priests. This rite occupied only one day, with three pressings of soma, at morning, noon and evening; but this day was preceded by very detailed preparatory ceremonies, one of which was initiation (diksha) of the sacrificer and his wife other
soma sacrifices lasted for several days upto twelve; while another class, called sattras or "sessions", extended to a year or more.
A very sacred ceremony that can be connected with the soma sacrifice is the Agni chayana, or "piling of the fire-altar", which lasts for a year. It begins with the sacrifice of five animals. Then a long time is occupied in preparing the earthenware vessel, called ukha, in which fire is to be maintained for a year. Very elaborate rules are given both as to the ingredients, such as the hair of a black antelope, with which the clay is to be mixed, and as to how it is to be mixed, and as to how it is to be shaped and finally burnt. Then, the bricks, which have different and particular sizes have to be built up, as its bricks are placed in position, to the accompaniment of appropriate rites and verses, by a formidable array of priests.
The shrauta sutra of ashvalayans which consists of twelve books, is related to Aitareya Brahmana.