प्रथमयोः पूर्वसवर्णः

For the simple vowel of a nominal-stem and for the vowel of the case-affixes of the Nominative and the Accusative in all numbers, there is the single substitution of a long vowel corresponding to the first vowel.
The words अकः and दीर्घः are both understood here. The word प्रथमा here means the प्रथमा विभक्तिः i. e. the Ist case or the Nominative, and includes here the द्वितीया विभक्ति also. This sutra teaches the substitution of a पूर्वरूप or a homogenous long vowel corresponding to the first vowel. Thus अग्नि + औ = अग्नी; वायु + औ = वायू; वृक्ष + अस् = वृक्षाः, so also प्लक्षाः, वृक्षान्, प्लक्षान् ॥ In the case of these last four examples (वृक्ष + अस् &c) the rule 6.1.97 would have caused para-rupa substitution i. e. would have given the form वृक्ष + अस् = वृक्षः, and that rule would have debarred the lengthening rule 6.1.101 on the maxim "apavadas that precede the rules which teach operations that have to be superseded by the apavada operations, supersede those rules that stand nearest to them;" but not this पुर्वसवर्ण lengthening rule as it does not stand nearest. The word अचि 'when a vowel follows' is understood here also. Thus वृक्ष + स् = वृक्षः, प्लक्षः ॥ The word अक् 'the simple vowel' is understood here also. Thus नौ + औ = नावौ ॥ Why do we say 'a vowel homogenous to the antecedent?' The substitute will not be one homogenous to the second or the subsequent vowel. The दीर्घ or 'long' is used in order to debar the substitution of pluta vowel having 3 measures for a pluta vowel.