Śatapatha Brāhmaṇa ("of a hundred paths") 3.6.1.3

3.6.1.3

atha ya eṣa madhyamaḥ śaṅkurbhavati | vederjaghanārdhe tasmātprāṅ prakrāmati ṣaḍvikramāndakṣiṇā saptamamapakrāmati sampadaḥ kāmāya tadavaṭam parilikhati

From the peg which stands in the middle on the hind-part of the altar, he strides six steps eastwards (along the 'spine'); the seventh he strides away from it to the right, for the sake of completeness, and there marks off a pit.