न च्छन्दस्यपुत्रस्य

Operational rule vidhi

In the छन्दस् (Vedas) the above rules causing lengthening or the substitution of long ई for the final vowel of the अङ्ग (stem), do not apply, except in the case of पुत्र.

Word by word (padaccheda)

Indeclinable न avyayaRight context छन्दसि saptamī · ekavacanaTarget अपुत्रस्य ṣaṣṭhī · ekavacana

Indeclinable An avyaya such as च "and", वा "optionally", न "not".
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").
Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).

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Full reading

न छन्दसि अपुत्रस्य

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

Commentary

In the छन्दस् (Vedas) the above rules causing lengthening or the substitution of long ई for the final vowel of the अङ्ग (stem), do not apply, except in the case of पुत्र।
Thus मित्रयुः, संस्वेदयुः, देवाञ् जिगाति सुम्नुयुः ॥ But पुत्रीयन्तः, सुदानवः (Rig VII.96.4).

Vart:- It should be rather stated पुत्र and the rest: as जनीयन्तोऽन्वयः See 3.2.170, for the affix उ ॥

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