Yellow and white represent the gold and silver keys of Saint Peter. The crossed keys represent the Papacy's spiritual and worldly power, and the papal tiara tops them.
History
Adopted on June 7, 1929, when Pope Pius XI signed the Lateran Treaty with Italy, establishing Vatican City as an independent state.
Construction
A 1:1 ratio (square). A vertical bicolour of yellow (hoist) and white (fly), with the crossed keys of Saint Peter and the papal tiara centered in the white band.