An in-depth analysis of the semantics of aggregation in object modelling leads to the identification of the primary axioms of whole-part; irreflexivity at the instance level; antisymmetry at both instance and type level; and in which the aggregate object has one or more emergent and resultant properties. Other aspects of aggregation are either a natural consequence of these axioms or else are secondary. Each secondary characteristic defines a partition and thus gives the modelled aggregation relationship a specific "flavour".