This paper presents CMod, a novel tool that provides a sound module system for C. CMod works by enforcing a set of four rules that are based on principles of modular reasoning and on current programming practice. CMod's rules flesh out the convention that .h header files are module interfaces and .c source files are module implementations. Although this convention is well-known, developing CMod's rules revealed there are many subtleties in applying the basic pattern correctly. We have proven formally that CMod's rules enforce both information hiding and type-safe linking. We evaluated CMod on a number of benchmarks, and found that most programs obey CMod's rules, or can be made to with minimal effort, while rule violations reveal brittle coding practices including numerous information hiding violations and occasional type errors.