एचोऽयवायावः

Operational rule vidhi

When followed by a स्वर, the letters of the एच् प्रत्याहार are respectively converted to अय्, अव्, आय् and आव् in the context of संहिता.

Word by word (padaccheda)

Target ṣaṣṭhī · ekavacanaSubstitute / term अय्- अव्- आय्- आवः prathamā · bahuvacana

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.

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

एचोऽयवायावः Diphthongs before a vowel

एच्in place of		आदेशsubstitute
ए	→	अय्
ओ	→	अव्
ऐ	→	आय्
औ	→	आव् written आवः

Why it is a table

1.3.10 यथासंख्यमनुदेशः समानाम् Two lists of equal length pair up in order (यथासङ्ख्यम्), so ए → अय्, ओ → अव्, ऐ → आय्, औ → आव् (the Kāśikā: आदेशा यथासंख्यं भवन्ति).

Checked against vidyut-prakriya derivations.

Full reading

अय्-अव्-आय्-आवः

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)

→ अय्- अव्- आय्- आवः

Replace ecaḥ with ay-av-āy-āvaḥ. Inherited words (anuvṛtti) may add further conditions.

Commentary

For the vowels ए ऐ ओ and औ are respectively substituted अय् आय् अव् and आव् when a vowel follows.
Thus चि+ ल्युट् = चे + अन = चयनम्, लो + अन् = लवनं; चै + अक = चायकः, लौ + अक =लावकः ॥ So also कयेते, व्ययेते, याववरुणद्धि, रु + युच् 3.2.148 = रो + अन = रवणः ॥

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