This paper reports on an approach for improving the understanding of old programs which have become very complex due to numerous extensions. We have adapted partial evaluation techniques for program understanding. These techniques mainly use propagation through statements and simplifications of statements. We focus here on the automatic interprocedural analysis and we specify both tasks (propagation and simplification) for call-statements, in terms of inference rules with notations taken from the formal specification languages B and VDM. We describe how we have implemented that interprocedural analysis in a tool, and how it can be used to improve program understanding. The difficulty of that analysis is due to the lack of well defined interprocedural mechanisms and the complexity of visibility rules in Fortran.