On December 7, 1787, Delaware became the first state to ratify the U.S. Constitution and thus the first state of the United States of America. The 13-cent Delaware State Flag (Scott 1633) is hence the first stamp on the pane of fifty state flags, appearing in the upper left corner of the pane.
The state flag of Delaware was adopted on July 24, 1913. A field of colonial blue is the setting for a buff-colored diamond within which the coat-of-arms of the state—depicting early occupational symbols for shipping, farming, hunting, and cattle ranching—is displayed. Beneath the diamond and the coat-of-arms is the date of ratification, December 7, 1787.
The colonial blue field and the buff-colored diamond represent the colors of a uniform worn by General George Washington. The diamond is a reference to an early state nickname, the 'Diamond State', so given because of Delaware's small size and great value.
Scott 2005 Specialized Catalogue of U.S. Stamps and Covers