The entities constructed by programming are extremely complex. Accurate construction of large programs would be impossible without specific techniques for controlling this complexity. Most such techniques are based on finding ways to decompose a problem into almost independently solvable subproblems, allowing a programmer to concentrate on one subproblem at a time, ignoring the others. When the subproblems are solved, the programmer must be able to combine the solutions with a minimum of unanticipated interactions. To the extent that a decomposition succeeds in breaking a programming problem into manageable pieces, we say that the resulting program is modular; each part of the solution is called a Module. Well-designed programing languages provide features which support the construction of modular programs .
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⇒ The Art of the Interpreter, p. 1