तयोर्दार्हिलौ च च्छन्दसि

Operational rule vidhi

After the two प्रातिपदिक (Nominal Stems) इदम् and तद् come respectively the affixes दा and र्हिल् in the छन्दस् (Vedas) and also other affixes.

Word by word (padaccheda)

Target तयोः ṣaṣṭhī · dvivacanaSubstitute / term दा- र्हि औ prathamā · dvivacanaIndeclinable च avyayaRight context छन्दसि saptamī · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
Substitute / term Prathamā (1st case): in operational rules, the substitute or the thing introduced; in definitions, the term or its referent.
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").

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

Pink letters are it-markers (anubandhas), tags dropped by 1.3.9. Hover or tap one for the rule that marks it, or try the it-letter finder.

Full reading

तयोः दा-र्हिलौ च छन्दसि

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)

तयोः → दा- र्हिलौ / छन्दसि

Replace tayoḥ with dā-rhilau when chandasi follows. Inherited words (anuvṛtti) may add further conditions.

Commentary

After the two प्रातिपदिक (Nominal Stems) इदम् and तद् come respectively the affixes दा and र्हिल् in the छन्दस् (Vedas) and also other affixes.
The affix दा comes after इदम् and र्हि after तद् ॥ Thus इदा 5.3.3, तर्हि 7.2.102. So also इदानीम् and तदानीम् ॥ As इ॒दा हि य॒मुप॑स्तुतिभि॒दा वा॒मस्य॑ भ॒क्तये॑ (Rig Veda VIII.27.11).

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