The temporal evolution of three-dimensional instabilities on a planar liquid sheet segment is studied using direct numerical simulation, and the level-set and volume-of-fluid methods for the liquid-gas interface tracking. Three atomization cascades are distinguished at early breakup, which are well categorized on a gasWeber number (Weg) versus liquid Reynolds number (Rel) map. These atomization processes include lobe stretching that occurs at low Rel and low Weg, hole and bridge formation that occurs at moderate Rel and high Weg, and lobe corrugation occurring at high Rel and low Weg. Qualitative comparison between the sizes of the ligaments and droplets that result from each process is presented. A transitional region between the prescribed atomization domains is found. At high Rel , the transitional boundary is a constant Ohnesorge line defined based on gas We and liquid Re (Ohm √Weg=Rel). At low Rel , the transitional region follows a hyperbolic line on the Weg-Rel plot. These atomization processes are qualitatively independent of the jet geometry-seen in both planar and round liquid jets. At a constant density ratio, characteristic times for the hole formation and for the lobe and ligament stretching are different-the former depending on surface tension and the latter on liquid viscosity. In the transitional region, both characteristic times are of the same order. Published by AIP Publishing.