Before a taddhita affix with an indicatory ñ , ṇ or k the vṛddhi is not substituted for the first vowel in a compound when it follows a word ending in y or v but ai and au are respectively placed before the semi-vowels.,
That is ऐ is placed before य्, and औ before व् ॥ As वैयसनम् from व्यसन; (व्यसने भवं) वैयाकरणः from व्याकरणं (व्याकरणमधीते) सौवश्वः from स्वश्वः (स्वश्वस्यापत्यं) ॥ Why after य् or व् only? Observe त्रार्थिः son of त्रर्थः ॥ Why do we say 'य् or व् final of a pada or word'? Observe याष्टीकः from यष्टिः, (यष्टिः प्रहरणमस्य) 4.4.59 याता from यति (यतेश्छात्राः or यत इमे छात्राः) ॥ The rule does not apply to दाध्यश्वि and माध्वश्वि, for no rule ordains the Vriddhi of ध्य the ध्व, and so no occasion for the prohibition of this rule arises. These are Patronyms formed by इञ् 4.1.95 from दध्यश्व and मध्वश्व (दधि प्रियोऽश्वोयस्य &c). The present rule applies to that Vriddhi also which takes place in the second member of the compound by 7.3.10 &c. As पूर्वत्रैयालिन्दः from पूर्वत्र्यलिन्द (पूर्वत्र्यलिन्दे भवः) ॥ But this prohibition does not apply where the य् or व् are not the parts of the second member, as द्व्याशीतिकः (= द्वे अशीती भूतो, भूतो भावी वा) ॥,