Agile Principles, Patterns, and Practices in C#

The Open/Closed Principle is at the heart of many of the claims made for object-oriented design. When this principle is in effect, applications are more maintainable, reusable, and robust. The Liskov Substitution Principle is one of the prime enablers of OCP. The substitutability of subtypes allows a module, expressed in terms of a base type, to be extensible without modification. That substitutability must be something that developers can depend on implicitly. Thus, the contract of the base type has to be well and prominently understood, if not explicitly enforced, by the code.

The term IS-A is too broad to act as a definition of a subtype. The true definition of a subtype is substitutable, where substitutability is defined by either an explicit or implicit contract.