A striking feature in bivalved seashells is that the 2 valves fit together perfectly when closed. This trait has evolved in 2 phyla from a common shell-less ancestor and has been described for hundreds of years. While its functional advantage is clear, there is no understanding of how this feature is generated. A mathematical model of the shell growth process explains how geometry and mechanics conspire to generate an interlocking pattern. This model provides a physical explanation for a prominent example of convergent evolution. By showing how variations in the mechanism create a wide variety of morphological trends the model provides insight into how biophysical processes, probably modulated by genetic factors, are manifest across scales to produce a predictable pattern.