Shatapatha Brahmana 7.3.1.27

7.3.1.27

athottaravediṃ nivapati | iyaṃ vai vedirdyairuttaravedirdiśo logeṣṭakāstadyadantareṇa vediṃ cottaravediṃ ca logeṣṭakā upadadhātīmau tallokāvantareṇa diśo dadhāti tasmādimau lokāvantareṇa diśastāṃ yugamātrīṃ vā sarvataḥ karoti catvāriṃśatpadāṃ vā yatarathā kāmayetātha sikatā nivapati tasyokto bandhuḥ

He then throws up the Uttara-vedi (high-altar),--the Vedi is this (earth), the Uttara-vedi the sky, and the clod-bricks are the regions: thus when he puts on the clod-bricks between (the preparation of) the Vedi and (that of) the Uttara-vedi, he thereby places the regions between these two worlds; whence the regions are between these two worlds. He makes it either a yoke long on each side, or forty feet,--whichever way he pleases. He then throws sand thereon: the meaning of this has been explained.