The Vedanta Sutras, commentary by Sankaracharya (SBE34), tr. by George Thibaut  at sacred-texts.com
32. Moreover they (the Gâbâlas) speak of him
(the highest Lord) in that (i.e. the interstice between the top of the head and the chin which is measured by a span).
Moreover the Gâbâlas speak in their text of the highest Lord as being in the interstice between the top of the head and the chin. 'The unevolved infinite Self abides in the avimukta (i.e. the non-released soul). Where does that avimukta abide? It abides in the Varanâ and the Nâsî, in the middle. What is that Varanâ, what is that Nâsî?' The text thereupon etymologises the term Varanâ as that which wards off (vârayati) all evil done by the senses, and the term Nâsî as that which destroys (nâsayati) all evil done by the senses; and then continues, 'And what is its place?--The place where the eyebrows and the nose join. That is the joining place of the heavenly world (represented by the upper part of the head) and of the other (i.e. the earthly world represented by the chin).' (Gâbâla Up. I.)--Thus it appears that the scriptural statement which ascribes to the highest Lord the measure of a span is appropriate. That the highest Lord is called abhivimâna refers to his being the inward Self of all. As such he is directly measured, i.e. known by all animate beings. Or else the word may be explained as 'he who is near everywhere--as the inward Self--and who at the same time is measureless' (as being infinite). Or else it may denote the highest Lord as him who, as the cause of the world, measures it out, i.e. creates it. By all this it is proved that Vaisvânara is the highest Lord.