Śatapatha Brāhmaṇa ("of a hundred paths") 13.3.3

Inasmuch as there are three Anushtubh verses (on the first day), therefore the horse, when standing, stands on three (feet); and inasmuch as (they are made into) four Gâyatrî verses, therefore the horse, when stepping out, scampers off on all (four) feet. For that Anushtubh, doubtless, is the highest metre, and the horse is the highest of animals; and the Katushtoma is the highest of Stomas: by means of what is highest he thus causes him (the Sacrificer) to reach the highest position.

The Sakvarî verses are the (Hotri's) Prishtha (of the second day): there is a different metre for each (verse), for different Stomas are performed on each (day). And as to the Sakvarî verse being the Prishtha (-stotra), it is for the completeness of the horse (sacrifice).

The central day is an Ekavimsa one, for the Ekavimsa is yonder sun, and so is the Asvamedha by means of its own Stoma he thus establishes it in its own deity.

The Vâmadevya is the Maitrâvaruna's Sâman; for the Vâmadevya is Pragâpati, and the horse is of Pragâpati's nature: he thus supplies it with its own deity.

The Pârthurasma is the Brahma-sâman; for the horse is restrained by means of reins (rasmi), but when unrestrained, unchecked, and unsteadied, it would be liable to go to the furthest distance: thus when the Pârthurasma is the Brahma-sâman, it is for the safe keeping of the horse.

The Samkriti is the Akhâvâka's Sâman;-- that Asvamedha, indeed, is, as it were, a disused sacrifice, for what is performed thereof, and what is not? When the Samkriti is the Akhâvâka's Sâman, it is for (bringing about) the completeness of the horse (sacrifice). The last day is an Atirâtra with all the (six) Stomas, in order to his (the Sacrificer's) obtaining everything, for an Atirâtra with all the Stomas is everything, and the Asvamedha is everything.

The fire-altar is the twenty-one-fold one, the Stoma the twenty-one-fold one, and there are twenty-one sacrificial stakes; even as bulls or stallions would clash together, so do these Stomas, the twenty-one-versed, run counter to one another: were he to bring them together, the Sacrificer would suffer harm, and his sacrifice would be destroyed.

There may, indeed, be a twelvefold altar, and eleven stakes. When the altar is a twelvefold one--twelve months being a year--it is the year, the sacrifice, he obtains. When there are eleven stakes, then that Virâg (metre), the Ekâdasinî, is contrived; and that which is its eleventh (stake) is its teat: thereby he milks it.

As to this they say, 'If there were a twelvefold altar, and eleven stakes, it would be as if one were to drive on a cart drawn by one beast.' There are the twenty-one-fold altar, the twenty-one-fold Stoma, and twenty-one stakes: that is as when one drives with side-horses.

That twenty-one-fold one, indeed, is the head of the sacrifice; and, verily, he who knows three heads on the Asvamedha, becomes the head of kings. There are the twenty-one-fold altar, the twenty-one-fold Stoma, and twenty-one stakes: these are the three heads on the Asvamedha; and, verily, he who thus knows them becomes the head of kings. And, indeed, he who knows the three tops on the Asvamedha, becomes the top of kings;--there are the twenty-one-fold altar, the twenty-one-fold Stoma, and twenty-one stakes: these, indeed, are the three tops on the Asvamedha; and, verily, he who thus knows them becomes the top of kings.