/pitR- / पितृॱ

The base **pitR- पितृॱ m plus non-/sambuddhi /su makes **pitA पिता by Rduzanasp... —

/pitR- + /su → **pitA पिता

It becomes pitar- पितर् before other strong and before /Gi because of RtoGisarv... —

/pitR- + /sambuddhi → **pitar पितर्

/pitR- + /am → **pitaram पितरम्

/pitR- + /au → **pitarau पितरौ

/pitR- + /jas → **pitaras पितरस्

/pitR- + /Gi → **pitari पितरि

Oddly, R ऋ merges with /Gas /Gasi into ur उर् —

/pitR- + /Gasi → **pitur पितुर्

/pitR- + /Gas → **pitur पितुर्

Elsewhere nothing special happens. We get our ikoyaNaci before vowels, just like after **RSi- ऋषिॱ or **guru- गुरुॱ —

/pitR- + /TA → **pitrA पित्रा

/pitR- + /Ge → !**pitre पित्रे

and the R ऋ stays before consonants —

and before /zas and /nAm it works like the other vowels —

The base **mAtR- मातृॱ f works like **pitR- पितृॱ, except that of course there is no tasmAcchason... —

**mAtR- मातृॱ + /zas → **mAtRRs मातॄस्

The base **svasR- स्वसृॱ f works like **pitR- पितृॱ, except for the aptRntRcsvas... lengthening q.v.

It has to be mentioned that the long RR ॠ sound only happens in the natural language when rules prathamayoHp... or nAmi lengthen an R ऋ. As Coulson beautifully puts it, "RR ॠ is not part of inherited PIE phonology."