Natyashastra
मध्यमोत्तमयोः कार्या चतुरश्रावसानिकी । मध्याधमानां कर्तव्या त्र्! यश्रा चैवावसानिकी ॥ २३॥
For superior and middling characters, the concluding Dhruvā should be in Caturasra metre, and for the inferior characters, it should be in Tryasra metre.
english translation
madhyamottamayoH kAryA caturazrAvasAnikI । madhyAdhamAnAM kartavyA tr! yazrA caivAvasAnikI ॥ 23॥
hk transliteration by Sanscript