Natyashastra
त्र्! यश्रं वा चतुरश्रं वा योगं ज्ञात्वा प्रयोगजम् । तेन प्रमाणयोगेन ध्रुवा कार्याऽवसानिकी ॥ १९॥
Having understood the combination of the three-beat or four-beat measure arising from practical use, one should compose the Dhruvā in accordance with that measure as its concluding form.
english translation
tr! yazraM vA caturazraM vA yogaM jJAtvA prayogajam । tena pramANayogena dhruvA kAryA'vasAnikI ॥ 19॥
hk transliteration by Sanscript