On the Criteria To Be Used in Decomposing Systems into Modules Alive

Parnas compared two decompositions of the same KWIC index program: the traditional flowchart-derived one collapsed under change; one decomposed by 'design decisions likely to change' — each module hiding one secret — survived it.

The paper founded information hiding as design principle: module interfaces should expose what is stable and hide what is volatile. It is the intellectual root of encapsulation, abstract data types, and API design — and Parnas later argued it applied equally to hiding what you don't yet know.