The benefits of object metamodeling are the clear and precise definition of a semantics for the modelling constructs in an object modeling language. Although this work has already been done for OML and UML, some problems remain concerning the Whole-Part relationship, also called aggregation. This paper introduces a new style of object metamodeling in order to build a general yet abstract metatype called Whole-Part(Whole, Part). Subtypes are then defined according to the possibility or not to mix properties generally assigned to aggregation in the literature. All the candidate properties are partitioned into three disjoint sets: the set of primary features owned by the Whole-Part(Whole, Part) metatype, the set of implied properties resulting from the adoption of the primary properties, as well as the set of secondary features. Several "flavors" of aggregation are then constructed leading to reliable and consistent types of the Whole-Part relationship.