A paper by D.L. Parnas, Carnegie-Mellon University.

The Association for Computing Machinery (ACM) have a wonderful website that publishes the "classics" of Computer Science - defining papers published in their journal (Communications of the ACM).

This one (http://www.acm.org/classics/may96/, from Communications of the ACM, Vol. 15, No. 12, December 1972) deals with modularisation of a system. The "killer" paragraph reads:

We have tried to demonstrate by these examples that it is almost always incorrect to begin the decomposition of a system into modules on the basis of a flowchart. We propose instead that one begins with a list of difficult design decisions or design decisions which are likely to change. Each module is then designed to hide such a decision from the others. Since, in most cases, design decisions transcend time of execution, modules will not correspond to steps in the processing. To achieve an efficient implementation we must abandon the assumption that a module is one or more subroutines, and instead allow subroutines and programs to be assembled collections of code from various modules.
(My emphasis.)

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