Variation in modern maize relates to recent and ancient cultural events. Large complexes of maize races are usually characteristic of certain cultural regions and help to define boundaries and histories. Most of the regions were established 2000 yr ago; clear differentiation of maize must have also occurred by then. Racial complexes [24], numerous sub-complexes, and 6 regions are defined here and elsewhere. Most of the complexes seem descended from 7 ancestral types plus continual teosinte introgression. Spreading and interaction of these ancient types has had as important consequences as their isolation within regions.