सो ऽस्य-आदिर् इति च्छन्दसः प्रगाथेषु

The words of this sutra require a little explanation. सः 'that' i.e. a word in construction in the first case. अस्य 'its' i.e. this gives the meaning of the affix. आदिः 'beginning'; this qualifies the base or prakriti. The word इति is added for the sake of precision. छन्दसः 'after the name of a metre'. This points out the base. प्रगाथेषु 'in the sense of Pragatha'. This qualifies the meaning of the affix. To put it in other words, the sutra means, that the affix अण् is added to the name of a metre, when that metre is the beginning of a Pragatha, and the whole word so formed means a Pragatha beginning with that metre. Thus पंक्ति is the name of a metre, adding अण् to it we get पांक्तः which means \a Pragatha beginning with the metre Pankti\. Similarly आनुष्टुभः, जागतः &c.

Why do we say \beginning\? If the metre is in the middle or end of a 'Pragatha' the affix will not be employed. Why do we say 'after the name of a metre'? The affix will not be added to a word which is in the beginning of a Pragatha. Why do we say 'in the sense of a Pragatha'? The affix will not be added, if the metre is the beginning of an Anuvaka &c and not of a Pragatha. Pragatha is the name of a particular kind of Mantra or Hymn यत्र द्वे ऋचौ प्रगथनेन तिस्रः क्रियन्ते, स प्रगायनात् पकर्षगानात् वा प्रगाथ इत्युच्यते ॥

Varti:- The affix अण् is added to the name of a metre in a self-descriptive sense without changing the meaning of it; and the word so formed is in the neuter gender. As, त्रैष्टुभम् 'the metre Trishtubha', जागतम् &c.,