In this episode, we look at the Liskov Substitution Principle. If S is a subtype of T, then objects of type T may be replaced with objects of type S without altering any of the desirable properties of the program.

Open/Closed principle states "software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification"; that is, such an entity can allow its behaviour to be extended without modifying its code.

A class should only have a single responsibility, that is, only changes to one part of the software's specification should be able to affect the specification of the class.