An atomic-scale simulation model for anisotropic wet chemical etching of Si(1 0 0)-wafers covered with masks of arbitrary shape and a series of experiments for comparison are presented. The model assumes that the probability of removal of a surface atom depends on the number of first and second neighbors. A removal probability function is presented in order to describe the probabilities of removal corresponding to the different surface atoms having different numbers of first and second neighbors. Etching experiments on Si (1 0 0)-wafers using 10 wt.% KOH solution at 75 degreesC are presented and the performance of the model is evaluated against them. We compare the underetched structures obtained in the simulations and experiments using a mask pattern consisting of a wagon wheel and a set of rectangular frame-like openings with varying orientation. The simulations show good agreement with the experiments. The model predicts the existence of fastest-etched planes in accordance with experiment, and describes accurately the evolution of under-etching below the masks for all mask orientations, including the slopes of the planes appearing below the mask. The results show that the cooperative effects of atoms evolving according to a simple rule generate most features of the meso- and macroscopic etching patterns. They also show that the use of only first neighbors or a partial incorporation of the second neighbors in the modeling strategy is not sufficient in order to describe the under-etching processes and that the second neighbors must be fully incorporated into the model.