The authors consider constraints from axisymmetric, nearly inviscid theory on Hadley cell emergence and extent in dry planetary atmospheres. Existing versions of the well-known Hide's constraint relating Hadley cell emergence to the distributions of absolute angular momentum (M) and the vertical component of absolute vorticity (η) are unified, amounting to any of M > Ωa^2, M u_(amc) condition provides a simple explanation for why cross-equatorial Hadley circulations typically extend as far into the winter- as the summer hemisphere. The classical "equal-area" models predict φ_a but typically must be solved numerically and always predict φ)a at or poleward of the RCE forcing maximum (φ_m) for φ_m ≠ 0. In an idealized dry general circulation model, a pole-to-pole cross-equatorial Hadley cell emerges if the corresponding RCE state meets some combination of these extent criteria over the entire summer hemisphere. Conversely, the cell edge and φ_a sit far equatorward of φ_m if those criteria are not satisfied near φ_m
