Shatapatha Brahmana 7.5.1.23

7.5.1.23

tatprādeśamātram bhavati | prādeśamātramiva hi śiraścatuḥsrakti bhavati catuḥsraktīva hi śiro madhye saṃgṛhītam bhavati madhye saṃgṛhītamiva hi śiraḥ

It (the mortar) is of the measure of a span, for the head is, as it were, of the measure of a span;--four-cornered, for the head is, as it were, four-cornered;--contracted in the middle, for the head is, as it were, contracted in the middle.