Natyashastra
द्विर्भावान्मध्यमस्येव ज्येष्थमित्यभिधीयते । मध्यमस्य विधनेन न्यसेच्च द्विगुणाः कलाः ॥ ६५॥
From the duplication arises what is called Jyeṣṭha, just like the Madhyama. By the rule of the Madhyama, one should apply it, but with the units of time doubled.
english translation
dvirbhAvAnmadhyamasyeva jyeSthamityabhidhIyate । madhyamasya vidhanena nyasecca dviguNAH kalAH ॥ 65॥
hk transliteration by Sanscript