A derived or changed character state (i.e., an apomorphy) shared by two or more lineages in a particular clade. Synapomorphies are indicators of common ancestry. For example, within the clade of terrestrial vertebrates the ancestral, or plesiomorphic, character state is “has four legs.” However, both owls and parrots have the synapomorphic character state “has two legs and two wings,” indicating that owls and parrots are closely related.