6.3.9 हलदन्तात् सप्तम्याः संज्ञायाम्

Word by word (padaccheda)

Left context हलदन्तात् pañcamī · ekavacanaTarget सप्तम्याः ṣaṣṭhī · ekavacanaRight context संज्ञायाम् saptamī · ekavacana

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
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").

Full reading

हलदन्तात् सप्तम्याः संज्ञायाम्

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

Commentary

The Locative -7th case ending is not elided after a stem ending in a consonant or a short अ when the compound is a Name.
Thus युधिष्ठिरः, त्वचिसारः, गविष्ठिरः ॥ Though गो does not end in a consonant, yet it retains its Locative ending by virtue of 8.3.95 (गवियुधिभ्यां स्थिरः) which shows by implication that गवि is governed by this rule. So also अ ending words: as अरण्येतिलकाः, अरण्येमाषकाः, वनेकिंशुकाः, वनेहरिद्रकाः, वनेबल्बजकाः, पूर्वाह्णेस्फोटकाः, कूपेपिशाचकाः ॥ Why do we say "after a word ending in a consonant or अ"? Observe नद्यां कुक्कुटिका = नदीकुक्कुटिका, भूम्यां पाशाः = भूमिपाशाः ॥ Why do we say 'when a Name'? Observe अक्षशौण्डः ॥

Vart:- The Locative ending is not elided after हृद् and दिव्; as : हृदिस्पृक्, दिविस्पृकः ॥

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