अवयसि ठंश्च

Operational rule vidhi

The affix ठन् comes after the word षण्मास when 'age' is Not meant.

Word by word (padaccheda)

Right context अवयसि saptamī · ekavacanaSubstitute / term prathamā · ekavacanaIndeclinable च avyaya

Right context Saptamī (7th case): "when X follows", the operation applies to what precedes (1.1.66 तस्मिन्निति निर्दिष्टे पूर्वस्य). Also a condition or domain ("in the sense of").
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
Indeclinable An avyaya such as च "and", वा "optionally", न "not".

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Full reading

अवयसि ठन् च

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

Commentary

The affix ठन् comes after the word षण्मास when 'age' is Not meant.
The च draws in the affix ण्यत् also. Thus षण्मासिकः or षाण्मास्यो रोगः 'a disease lasting six months'.

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