Classes can inherit from more than the single superclass from which they were created. This is called multiple inheritance. Classes designed to add a set of instance and class methods to other classes are called mixin classes, or simply mixins.

You can add mixin methods to an existing class by sending an INHERIT message or using the INHERIT option on the ::CLASS directive. In either case, the class to be inherited must be a mixin. During both class creation and multiple inheritance, subclasses inherit both class and instance methods from their superclasses.

Mixins are always associated with a base class, which is the mixin's first non-mixin superclass. Any subclass of the mixin's base class can (directly or indirectly) inherit a mixin; other classes cannot. For example, a mixin class created as a subclass of the Array class can only be inherited by other Array subclasses. Mixins that use the Object class as a base class can be inherited by any class.

To create a new mixin class, you send a MIXINCLASS message to an existing class or use the ::CLASS directive with the MIXINCLASS option. A mixin class is also an object class and can create instances of the class.