Conceptually, the effective method is built in three steps: First, the generic function builds a list of applicable methods based on the actual arguments it was passed. Second, the list of applicable methods is sorted according to the specificity of their parameter specializers. Finally, methods are taken in order from the sorted list and their code combined to produce the effective method.9

To find applicable methods, the generic function compares the actual arguments with the corresponding parameter specializers in each of its methods. A method is applicable if, and only if, all the specializers are compatible with the corresponding arguments.

Because all the arguments are checked against the corresponding specializers, they all affect whether a method is applicable. Methods that explicitly specialize more than one parameter are called multimethods; I’ll discuss them in the section “Multimethods.”

After the applicable methods have been found, the generic function machinery needs to sort them before it can combine them into an effective method. To order two applicable methods, the generic function compares their parameter specializers from left to right,10 and the first specializer that’s different between the two methods determines their ordering, with the method with the more specific specializer coming first.

Multiple inheritance slightly complicates the notion of specificity since the actual argument may be an instance of two classes, neither of which is a subclass of the other. If such classes are used as parameter specializers, the generic function can’t order them using only the rule that subclasses are more specific than their superclasses. In the next chapter I’ll discuss how the notion of specificity is extended to deal with multiple inheritance. For now, suffice it to say that there’s a deterministic algorithm for ordering class specializers.

Finally, an specializer is always more specific than any class specializer, and because only applicable methods are being considered, if more than one method has an **EQL** specializer for a particular parameter, they must all have the same **EQL** specializer. The comparison of those methods will thus be decided based on other parameters.