Air traffic hubs are not airports; they are geographical areas providing aviation services. An air traffic hub may be served by more than one commercial service airport (e.g., the Washington, DC, hub is served by Reagan National and Dulles International Airports). A large air traffic hub is an area with 1 percent or more of the total annual enplanements by all U.S. certificated air carriers. Medium hubs have between 0.25 and 0.99 percent of enplanements; small hubs have between 0.05 and 0.24 percent; and nonhubs have less than 0.05 percent of enplanements. This definition should not be confused with airline hub, defined below.