The affix ru comes after the verbs 1. dā 'to give' 2. dheṭ 'to suck' 3. si 'to bind' 4. śada 'to fall' 5. sad 'to sit'.,
As, दारुः 'munificent man'; धारुर्वत्सो मातरम् 'a sucking child of the mother'. The genitive case is here prohibited by 2.3.69, the रु being taken as equivalent to उ. As सरुः 'binding'; शद्रुः 'falling'; and सद्रुः 'resting'.,