guṇa is substituted for the final ṛ of a aṅga (stem) in the Locative -7th case singular and in the strong cases.,
As मातरि, पितरि, भ्रातरि, कर्तरि; मातरौ, पितरौ, भ्रातरौ, कर्तारौ, कर्तारः ॥ For the long in कर्तारौ and कर्तारः see 6.4.11. The त् in ऋत् is for the sake of facility of utterance.
Because before ङि and strong cases, it is impossible for a stem to end in a long vowel ॠ nor a dhatu noun can so come, since in that case, long ॠ would be changed to इर् by (VII. I. 100), and in forms like कर्तॄणि there is नुम् ॥,