Implementation of the nine-digit zip code : hearings before the Subcommittee on Civil Service, Post Office, and General Services of the Committee on Governmental Affairs, United States Senate, Ninety-seventh Congress, first session, on S.J. Res. 11, S. 539, and S. 678, February 24 and April 29, 1981. (U.S. G.P.O.,, 1981)