The types of all procedure parameters and return types must be specified in the procedure's header. However, to maximize the utility of parallel procedures, the precise sizes of parallel variables can be omitted when used in a procedure. In particular, rank-defined arrays specify the rank of a parallel variable without specifying upper and lower bounds for each dimension. Their actual bounds are bound when the region specifier of the appropriate rank becomes known at runtime. Local variables may also be declared as rank-defined arrays. Thus, procedures can be written to accept arguments of any size as long as their dimensions match.