8.2.108 तयोर्य्वावचि संहितायाम्

Word by word (padaccheda)

Of तयोः ṣaṣṭhī · dvivacanaTerm / subject य्वौ prathamā · dvivacanaIn / when saptamī · ekavacanaIn / when संहितायाम् saptamī · ekavacana

Of Ṣaṣṭhī (6th case): "of X". In this kind of sūtra it marks a relation, not a substitution.
Term / subject Prathamā (1st case): the term being defined, or the subject of the statement.
In / when Saptamī (7th case): "in X" or "when X", a domain or condition.

Dotted words are technical terms, dashed ones pratyāhāras (names for classes of sounds or affixes). Hover or tap one to see its card.

Full reading

तयोः य्वौ संहितायाम्

Pāṇini leaves out words that carry over from earlier sūtras. Expand to see the rule as a reader actually understands it.

Commentary

For these vowels इ or उ are substituted य् and व् when a vowel follows them in a संहिता (in an unbroken flow of speech).
Karika:-

किं तु यणा भवतीह न सिद्धं य्वाविदुतोर्यदयं विदधाति ।
तौ च मम स्वरसन्धिषु सिद्धौ शाकलदीर्घविधी तु निवर्त्यौ ॥
इक तु यदा भवति प्लुतपूर्वस्तस्य यणं विदधात्यपवादम् ।
तेन तयोश्च न शाकलदीर्घौ यण्स्वरबाधनमेव तु हेतुः ॥

The word संहितायाम् is an adhikara and exerts its influence upto the end of the Book. Whatever we shall teach hereafter, upto the end of the Book, will apply to words which are in Sanhita.

Thus अग्ना३याशा, पटा३वाशा, अग्ना३यिन्द्रम्, पटा३वुदकम् ॥
Why do we say संहितायाम् "when a vowel follows"? Observe अग्ना३इ, पटा३उ ॥

Why do we say संहितायाम् 'in an uninterrupted flow of speech'? Observe अग्ना३ इ इन्द्रम्, पटा३उ उदकम् in Pada Patha.

This sutra is made, because इ and उ being pluta-modifications are considered as asiddha 8.2.1 for the purposes of इको यणचि 6.1.77. But supposing that some how or other, these इ and उ be considered siddha, still the present rule is necessary to prevent their lengthening before a homogeneous vowel 6.1.101, or their retaining their form unchanged by 6.1.127. If it be said, that these rules 6.1.101, and 6.1.127, would not apply, because of the Vartika इकः प्लुतपूर्वस्य सवर्णदीर्घ बाधनार्थे यणादेशो वक्तव्यः (Vartika to 6.1.77); still the present sutra ought to be made, in order to prevent यण् accent 8.2.4.

Karika:- Question: Could not this have been accomplished by the यणादेश of 6.1.77, that the Acharya has taught this separate य् and व् substitution ?

Answer:- If you say that pluta is siddha in the vowel sandhi rules, because of the jnapaka in sutra 6.1.125, then still this rule is necessary in order to prevent the lengthening of 6.1.101, and the Sakala rule 6.1.127.

Question:- But those two rules 6.1.101, and 6.1.127, have already been debarred by the Vartika which ordains that यणादेश takes place of such a vowel following a pluta (See vartika to 6.1.77), what is then the necessity of the present sutra ?

Answer:- The necessity of the present sutra is to prevent the यण-accent of 8.2.4.

According to one view, the vartika under 6.1.77, refers to those vowels which are not pluta-elements, like the present इ and उ, but which are independent vowels following after a pluta vowel. As भो३ इ इन्द्र = भो ३ यिन्द्र ॥

Here इ is a Nipata.

Śrīśa Chandra Vasu, The Ashtadhyayi of Panini (1897), via ashtadhyayi.com.