उपसर्गाद् बहुलम्

The न of नस् is changed diversely into ण , when it comes after an उपसर्ग having a cause of change.
Thus प्रणः शूद्रः, प्रणसः प्रणो राजा ॥ The change does not sometime take, place, as, प्र नो मुञ्चतम् ॥

By force of the word बहुलम् the rule applies to secular literature also as distinguished from the Chhandas or sacred Vedic text. Thus प्रणसं मुखं ॥ The word छन्दसि does not govern this sutra.

In प्रणसं मुखम्, the word नस् is the substitute for नासिका and means 'nose'. See 5.4.119.

This is not the sutra as given by Panini. His sutra is उपसर्गादनोत्परः "after an Upasarga, the न of नस् when not preceded by or followed by an ओ, is changed into ण्". There being several objections to this rule, the above rule is substituted as an amendment, in the Mahabhashya. For ओत्परः may have two meanings:-(1) ओकारात् परः, that which comes after ओ; (2) ओकारः परोऽस्मात् that which is followed by ओ ॥ In both cases, the rule is objectionable. In the first case, the ण change would be required in प्रनो मुञ्चतं; in the second case, there should be no ण change in प्रणोवनिर्देवता ॥ In fact, we find in the Vedas प्रणः as well as प्रनः, so also प्रणो as well as प्रनो ॥ Hence the necessity of this amendment.