All variables defined within an external subprogram exist only in that subprogram. In other words, the scope of a variable in an external subprogram is confined to the subprogram in which it is defined.

The limited scope of variables provides another benefit to programmers: it allows the programmer to reuse variable names in different subprograms. For example, in the Fortran example program above with the power function, both the main program and the function have a variable called exponent. The x in the main program and the example in the function are different variables, i.e. they represent different memory locations, and can therefore contain separate values. Another way of saying this is that each subprogram has its own name space. The names of variables in one subprogram are kept separately from the names of variables in other subprograms.