क्रमश्च क्त्वि

Operational rule vidhi

The उपधा letter of the verb-root 'क्रम्' undergoes an optional दीर्घादेश when followed by a झलादि क्त्वा प्रत्यय.

Word by word (padaccheda)

Target क्रमः ṣaṣṭhī · ekavacanaIndeclinable च avyayaRight context क्त्वि saptamī · ekavacana

Target Ṣaṣṭhī (6th case): names what gets replaced, "in place of X" (1.1.49 षष्ठी स्थानेयोगा).
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").

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Full reading

क्रमः च क्त्वि

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

Commentary

The penultimate of क्रम is optionally lengthened before the affix क्त्वा when it is without the augment इट्।
Thus क्रन्त्वा or क्रान्त्वा; but क्रमित्वा only, with इट् augment. But when क्त्वा is replaced by ल्यप्, the rule does not apply: as प्रक्रम्य, उपक्रम्य ॥ This is on the maxim अन्तरङ्गानपि विधीन् बहिरङ्गोल्यप् बाधते "a bahiranga substitution of ल्यप्' supersedes even antaranga rule".

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