प्राचां ष्फ तद्धितः

In the opinion of the Eastern Grammarians, the affix ष्फ is used after what ends with the affix यञ् and it is to be regarded as a तद्धित-affix.
Of this affix ष्फ the letter ष् is indicatory 1.3.6, and shows that the words formed by this affix will take 'nish' 4.1.41. The feminine nature is here manifested by these two affixes jointly, viz by 'shpha' and 'nish'. The object of saying that 'shpha' is to be regarded as a taddhita affix, is that the word so formed may get the designation of pratipadika (1.2. 46). The affix फ is replaced by the substitute आयन 7.1.2. Thus गार्ग्य + ष्फ + ङीष् = गार्ग्यायणी; so also वात्स्यायनी ॥ This being an optional rule, we have in the alternative, गार्गी and वात्सी ॥

The word सर्वत्र "every where", of the next sutra is to be read into this, by a process inverse to that of anuvritti, in order to prohibit the application of those rules even which would otherwise debar this. Thus 4.1.75 says:- 'the affix चाप् comes after आवट्य' ॥ Here आवट्य ends in यञ्, and by 4.1.16, the word आवट्य would have taken the affix nip: 4.1.75 debarred this. But according to Eastern Grammarians, here also the affix 'shpha' will be added. Thus आवट्यायनी ॥