The method of stepwise refinement led us on the data side to structured data types and on the algorithm side to procedures. Parameter lists turn procedures into a flexible, reusable construct. A procedure can therefore be used in multiple programs, as long as it is written in a sufficiently general way. The reusability of procedures requires that there are no changes or use of global variables, that all parameter types are known, and that the procedures are recompiled in the using program. Both the writer of the procedure and its user are thus required to exercise programming discipline, as a procedure is not always independent of its environment. These weaknesses are addressed by modules, which also bring the valuable property of allowing the decomposition (modularization) of the program into individual parts.