इकः वहेऽपीलोः

Operational rule vidhi

The final इ and the उ of a word with the exception of पीलु are lengthened before वह.

Word by word (padaccheda)

Target ṣaṣṭhī · ekavacanaRight context वहे saptamī · ekavacanaTarget अपीलोः ṣaṣṭhī · 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").

Dotted words are technical terms, dashed ones pratyāhāras (names for classes of sounds or affixes). Hover or tap one to see its card.

Full reading

वहे अपीलोः

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

Commentary

The final इ and the उ of a word with the exception of पीलु are lengthened before वह।
Thus ऋषीवहम्, कपीवहम्, मुनीवहम् ॥ Why do we say 'ending in इक् vowels'? Observe पिण्डवहम् ॥ Why not of पीलु? Observe पीलुवहम् ॥

Vart:- It should be stated "with the exception of पीलु and the rest." As दारुवहम् ॥

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