एतेतौ रथोः

Operational rule vidhi

The substitutes एत and इत come instead of इदम् when a प्राग्दिशीय affix beginning with र and थ respectively follows.

Word by word (padaccheda)

Substitute / term एत- prathamā · dvivacanaRight context र- थोः saptamī · dvivacana

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

एत- र-थोः

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

Commentary

The substitutes एत and इत come instead of इदम् when a प्राग्दिशीय affix beginning with र and थ respectively follows.
This debars इश् ॥ The अ in र is for the sake of euphony. Thus इदम् + र्हिल = एतर्हि 5.3.16; इदम् + थमु = इत्थम् 5.3.24.

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