पितरामातरा च च्छन्दसि

Operational rule vidhi

In the छन्दस् (Vedas) the form पितरामातरा is a valid form.

Word by word (padaccheda)

Substitute / term पितरामातरा prathamā · dvivacanaIndeclinable च avyayaRight context छन्दसि saptamī · ekavacana

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").

Full reading

पितरामातरा च छन्दसि

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

Commentary

In the छन्दस् (Vedas) the form पितरामातरा is a valid form.
In the ordinary language मातापितरौ is the proper form. The Vedic form is derived by adding अङ् to the first member, and आ is added to the second by 7.1.39 : and then Guna by 7.3.110. Thus आ मा गन्तां पितरामातरा च ॥

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