सर्वासामेव जातीनां त्रिविधं वृत्तमुच्यते । गुरुप्रायं लघुप्रायं गुरुलघ्वक्षरं तथा ॥ ३९॥
For all these classes of metres, three kinds of metrical forms (vṛtta) are prescribed: one consisting predominantly of long syllables, one predominantly of short syllables, and one composed of both long and short syllables.
english translation