The story of "Old Glory" - how did the stars and stripes earn it's famous name? → http://www.usflag.org/old.glory.story.html
A “canton” is the proper name for the blue field on which rest the 50 white stars representing the 50 states in our union. In international flag terminology, the 13 alternating red and white horizontal stripes and 50 white stars on a blue square in the upper left corner which together comprise the U.S. flag would be:"A banner Gules, six bars Argent; the canton Azure charged with 50 mullets Argent."
Red symbolizes Hardiness and Valor, White symbolizes Purity and Innocence and Blue represents Vigilance, Perseverance and Justice.