Abstract
A simple model consisting of three distinct dimer sublattices is proposed to describe the magnetism of NH4CuCl3. It explains the occurrence of magnetization plateaus only at 1/4 and 3/4 of the saturation magnetization. The field dependence of the excitation modes observed by electron-spin-resonance measurements is also explained by the model. The model predicts that the magnetization plateaus should disappear under high pressure.