A metamodel is a stereotyped model denoting that the model is an abstraction of another model, i.e., it is a model of a model. Hence, if M2 is a model of the model M1, then M2 is a metamodel of M1. It follows then that classes in M1 are instances of metaclasses in M2. The stereotype can be recursively applied, as in the case of a 4-layer metamodel architecture.