Natyashastra
तत्र युग्मं भवेद्द्व्यङ्गं श्रमेकाङ्गमेव च । प्रवृत्तमपि विज्ञेयं द्व्यङ्गमेकाङ्गमेव वा ॥ २६७॥
There, the Yugma (dual) should have two limbs, while the Tryasra (triple) should have one. Similarly, the Pravṛtta (middle section) also should be known as consisting of either two limbs or one.
english translation
tatra yugmaM bhaveddvyaGgaM zramekAGgameva ca । pravRttamapi vijJeyaM dvyaGgamekAGgameva vA ॥ 267॥
hk transliteration by Sanscript