सध मादस्थयोश्छन्दसि

Operational rule vidhi

सध is substituted for सह in the छन्दस् (Vedas) when माद and स्थ follow.

Word by word (padaccheda)

Word सध Right context माद- स्थयोः saptamī · dvivacanaRight context छन्दसि saptamī · ekavacana

Word Case not marked in the source data.
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

सध माद-स्थयोः छन्दसि

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

Commentary

सध is substituted for सह in the छन्दस् (Vedas) when माद and स्थ follow.
Thus सधमादोद्युम्न्य एकास्ताः, सधस्थाः ॥ Another example is आत्वा वृहन्तो हरयो युजाना, अर्वागिन्द्र सधमादो वहन्तु (Rig III.3.7) सहमाद्यन्ति देवा अस्मिन् ॥

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