Natyashastra
स्वभावरचिते भूमौ समस्थाने च यो भवेत् । समः पादः स विज्ञेयः स्वभावाभिनयाश्रयः ॥ २६९॥
The foot that is naturally placed evenly on the ground and stands in a balanced position is known as Sama (natural) foot, which forms the basis of natural expression.
english translation
svabhAvaracite bhUmau samasthAne ca yo bhavet । samaH pAdaH sa vijJeyaH svabhAvAbhinayAzrayaH ॥ 269॥
hk transliteration by Sanscript