Before the beginning of the Prâtaranuvâka (matin-chant), the sacrificer, sitting down behind the household altar (gârhapatya), and looking towards the north, sings the Sâman, addressed to the Vasus:

In plain words: Before the morning chant begins, the person making the offering sits behind the household fire altar, faces north, and sings a chant to the Vasus.