Normal form (pathyā)
Syllables 5-7 of the first pāda must be a ya-gaṇa ("⏑ – –")
General
The 1st and 8th syllables of both pādas are anceps.
The 2nd and 3rd syllables cannot both be light (laghu, "⏑") in either pāda; i.e. one or both of the 2nd and 3rd syllables must be heavy (guru, "–") in both pādas.
Syllables 2-4 of the second pāda cannot be a ra-gaṇa (the pattern "– ⏑ –")
Syllables 5-7 of the second pāda must be a ja-gaṇa ("⏑ – ⏑") This enforces an iambic cadence.
Variant forms (vipulā): The 4th syllable of the first pāda is heavy. In addition, one of the following is permitted:
na-vipulā: Syllables 5-7 are a na-gaṇa ("⏑ ⏑ ⏑")
bha-vipulā: Syllables 2-7 are ra-bha gaṇas ("– ⏑ – – ⏑ ⏑") or ma-bha gaṇas with a caesura in between ("– – – , – ⏑ ⏑")
ma-vipulā: Syllables 2-7 are ra-ma gaṇas with a caesura after the 5th ("– ⏑ – – , – –")
ra-vipulā: Syllables 5-7 are a ra-gaṇa following a caesura (", – ⏑ –")
Noteworthy is the avoidance of an iambic cadence in the first pāda. By comparison, Syllables 5-7 of any pāda in the old Vedic anuṣṭubh is typically a ja-gaṇa ("⏑ – ⏑"), or a dijambus.