Natyashastra
कार्यं द्वारद्वयं चात्र नेपथ्यगृहकस्य तु । पूरणे मृत्तिका चात्र कृष्णा देया प्रयत्नतः ॥ ६९॥
Here, two doors must be constructed for the stage, and the backstage (nepathya-gṛha) should be included. The floor should be made of black clay, prepared with effort.
english translation
kAryaM dvAradvayaM cAtra nepathyagRhakasya tu । pUraNe mRttikA cAtra kRSNA deyA prayatnataH ॥ 69॥
hk transliteration by Sanscript