Śatapatha Brāhmaṇa ("of a hundred paths") 8.4.4.1

8.4.4.1

athāto 'nvāvṛtam | trivṛdvatīm purastādupadadhātyekaviṃśavatīm paścātpañcadaśavatīṃ dakṣiṇataḥ saptadaśavatīmuttarataḥ

Now, then, as to the order of proceeding. That (brick) which contains the Trivrit (thrice-threefold stoma) he places in front, that containing the twenty-one-fold (stoma) at the back, that containing the fifteenfold (stoma) on the right (south) side, that containing the seventeenfold (stoma) on the left (north) side.