Natyashastra
अस्यैव समपादस्य पार्ष्णिरभ्यन्तरे भवेत् । त्र्यश्रपादः स विज्ञेयः स्थानकादिषु संश्रयः ॥ २७३॥
For the same evenly placed foot, the heel is inside; this is known as the tryashra foot, used in postures like standing.
english translation
asyaiva samapAdasya pArSNirabhyantare bhavet । tryazrapAdaH sa vijJeyaH sthAnakAdiSu saMzrayaH ॥ 273॥
hk transliteration by Sanscript