OOP allows subclasses to extend a class, both in terms of instance variables and methods.

A common practice it to let subclasses add instance variables; the subclass inherits all instance variables of its parent.

• Methods inherited from a superclass are guaranteed to work, since all needed instance variables will be available at the same constant offsets from the beginning of the record.

• Subclasses simply add variables to the end of the record.

• It is not possible to change an object from a superclass to one of its subclasses, since that would increase its size after it is allocated.

Multiple inheritance could cause problems of inconsistent offsets or name conflicts.

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