शेश्छन्दसि बहुलम्

Operational rule vidhi

In वेदाः, 'शि' is seen removed at some places.

Word by word (padaccheda)

Target शेः ṣaṣṭhī · ekavacanaRight context छन्दसि saptamī · ekavacanaSubstitute / term बहुलम् prathamā · ekavacana

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").
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.

Dotted words are technical terms. Hover or tap one to see its card.

Full reading

शेः छन्दसि बहुलम्

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

As a rewrite rule (sketch from case endings)

शेः → बहुलम् / छन्दसि

Replace śeḥ with bahulam when chandasi follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

In the छन्दस् , the elision of the case-ending इ (शि) of the nominative and accusative plural neuter, is optional.
Thus या क्षेत्रा or यानि क्षेत्राणि, या वना or यानि वनानि ॥

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