The subsumption relation can be understood as a relation of implication which relates more specific to more general concepts in conceptual taxonomies. In formal terms, subsumption defines a lattice, a kind of partial ordering, which may be represented as a directed acyclic graph. The hierarchical graphs defined by subsumption need not be trees, but can be more general kinds of graph in which child nodes are re-entrant, i.e. a child node may have more than one parent node. However, commonly a subsumption lattice has a core tree structure, with superimposition of more than one tree, or of other cross-classifiying structures. The subsumption relation may be seen as a generalisation relation, in that the subsumer expresses a generalisation over the subsumed.
Examples of lexical subsumption are shown in Figure 1, which illustrates some of the following points:
Figure 1: Reentrant subsumption graphs
Figure 2: Reentrant inheritance graphs