षष्ठ्याः पतिपुत्रपृष्ठपारपदपयस्पोषेषु

Operational rule vidhi

For the विसर्ग of the Genitive 6th-Case there is substituted स् in the छन्दस् (Vedas) before 1. पति 2. पुत्र 3. पृष्ठ 4. पार 5. पद 6. पयस् and 7. पोष.

Word by word (padaccheda)

Target षष्ठ्याः ṣaṣṭhī · ekavacanaRight context पति- पुत्र- पृष्ठ- पार- - पयस्- पोषेषु saptamī · bahuvacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
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").

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Full reading

षष्ठ्याः पति-पुत्र-पृष्ठ-पार- -पयस्-पोषेषु

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

Commentary

For the विसर्ग of the Genitive 6th-Case there is substituted स् in the छन्दस् (Vedas) before 1. पति 2. पुत्र 3. पृष्ठ 4. पार 5. पद 6. पयस् and 7. पोष।
Thus वाचस्पतिं विश्वकर्माणमूतये, (Rig. X. 81.7), दिवस्पुत्राय सूर्याय, दिवस्पृष्ठे धावमानं सुपर्णम्, अगन्म, तमसस्पारम्, इडस्पदे समिध्यसे, सूर्यं चक्षु र्दिवस्पयः, रायस्पोषं यजमानेषु धत्तम् ॥

Why do we say 'after a genitive case'? See मनुः पुत्रेभ्यो दायं व्यमजत् ॥

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