Pāṇini sūtras

ur is substituted for the final long ṝ of a root, when it is preceded by a labial consonant belonging to the root.

1. u+tउ+त्

   Part	Lexeme	Sandhi form	Meaning	Stem
   uउ	uउ	uउ	ukāraउकार	uउ
   tत्	tत्	tत्	TT	tत्

2. oṣṭhya+pūrvasyaओष्ठ्य+पूर्वस्य

   Part	Lexeme	Sandhi form	Meaning	Stem + termination
   oओ	oṣṭhyaओष्ठ्य	oṣṭhyaओष्ठ्य	labial consonant	oṣṭhyओष्ठ्य् + plus aअ
   pप्	pūrvaपूर्व	pūrvasyaपूर्वस्य	antecedent	pūrvaपूर्व + plus syaस्य

Anuvṛtti

1. dhātoḥधातोः 7·1·100

   Part	Lexeme	Sandhi form	Meaning	Stem + termination
   dद्	dhātuधातु	dhātoḥधातोः	Sanskrit verbal root	dhātधात् + plus oḥओः