Natyashastra
अत ऊर्ध्वं प्रवक्ष्यामि गीतानां वस्तुकेष्वपि । विवधैककवृत्तानि त्रीण्यङ्गानि समासतः ॥ १८०॥
I shall now explain the upper structure of the songs, even in the Vastus. The Vivadha, the Ekaka, and the Vṛtta together make up three limbs.
english translation
ata UrdhvaM pravakSyAmi gItAnAM vastukeSvapi । vivadhaikakavRttAni trINyaGgAni samAsataH ॥ 180॥
hk transliteration by Sanscript