Natyashastra
एवमेष प्रयोक्तव्यः पूर्वरङ्गो द्विजोत्तमाः । त्र्यश्रश्च चतुरश्रश्च शुद्धो भारत्युपाश्रयः ॥ १४८॥
Thus, O best among Brahmins, this prelude should be performed. It may be triangular or square, pure, and based on the Bhāratī style.
english translation
evameSa prayoktavyaH pUrvaraGgo dvijottamAH । tryazrazca caturazrazca zuddho bhAratyupAzrayaH ॥ 148॥
hk transliteration by Sanscript