The appendages of arthropods and vertebrates possess a third, proximodistal patterning axis that is established after the primary anteroposterior and dorsoventral body axes by mechanisms that are largely unknown. The vestigial gene is required for formation of the entire Drosophila wing, and the dorsal/ventral boundary is shown to organize wing formation and vestigial gene expression. Interactions between dorsal and ventral cells in the growing imaginal disc induce vestigial gene expression through a discrete, extraordinarily conserved imaginal disc-specific enhancer. The link between dorsal/ventral compartmentalization and wing formation distinguishes the development of this sheet-like appendage from that of legs and antennae.