Natyashastra
एवमष्टकलः कार्यः सन्निपातो विचक्षणैः । चत्वारः सन्निपाताश्च परिवर्तः स उच्यते ॥ ६३॥
Thus, the eight parts should be used in combination, with four combinations referred to as the "Sannipāta" (the union of rhythms).
english translation
evamaSTakalaH kAryaH sannipAto vicakSaNaiH । catvAraH sannipAtAzca parivartaH sa ucyate ॥ 63॥
hk transliteration by Sanscript