दिक्पूर्वपदाट्ठञ् च

Operational rule vidhi

And the affix ठञ् also (as well as यत्) comes in the remaining senses after the word अर्द्ध when the prior term denotes a direction.

Word by word (padaccheda)

Left context दिक्- पूर्वपदात् pañcamī · ekavacanaSubstitute / term prathamā · ekavacanaIndeclinable च avyaya

Left context Pañcamī (5th case): "after X", the operation applies to what follows (1.1.67 तस्मादित्युत्तरस्य). Also used for limits and sources.
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".

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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

दिक्-पूर्वपदात् ठञ् च

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

Commentary

And the affix ठञ् also (as well as यत्) comes in the remaining senses after the word अर्द्ध when the prior term denotes a direction.
By च, the affix यत् is also used. This debars अण् ॥ Thus पूर्वार्ध्यम्, पौर्वार्धिकम्, दक्षिणार्ध्यम् or दाक्षिणार्द्धिकम् ॥

The word पद is used in the sutra in order to prevent ambiguity. Had the sutra stood merely as दिक्पूर्वाट् ठञ्च, it might have meant 1.1.68, 'Ardha preceded by the word दिक् takes ठञ् also', for then स्वरूपविधि would have applied, and दिगर्धे जातः would have taken the affix.

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