In ancient times, the flag of the Papal States was evenly divided in two vertical stripes. The left side of the Holy See flag was red and the right side of the flag was yellow. These were the two colors of the Roman Senate and people.
It wasn’t until 1808 that the Papal States began to fly their own flag. In an effort to distinguish the Noble Guard of the Vatican from Napoleon’s army, Pope Pius VII ordered the guard to change the design of the Vatican flag. The flag was solid white with the Papal coat of arms at the center. This flag had been used by merchants since 1803. The coat of arms consisted of two keys, one silver and one gold, crossed diagonally with a red cord that ties the two together. The two keys represented the keys to heaven. The silver key represented the Popes power universal authority while the gold represents the religious strength. Setting atop these two keys is the papal tiara.
The solid white flag was used until 1825, at which time the solid white was broken in half. A vertical line of yellow was added to the left side of the flag to represent the universal power of the pope. The right side remained white to represent the religious strength. The seal was placed in the middle of the white stripe and the keys began to symbolize only the two keys to heaven. This flag was kept until 1849.
For a short time in 1849, the Italian red, white, and green flag flew. In the middle of the white stripe was the Roman motto “Dio e Popolo.” Pope Pius VII had been exiled and the Romans declared the Papal states were apart of the Roman States. However this only lasted from February until July, the white and yellow flag briefly flew again until 1870. At this time, Italy conquered the Papal States and the flag fell out of usage.
It was not until 1929, through the Lateran Treaty, that Italy declared that Vatican City was independent state. Vatican City adopted the flag of the Papal States and the square yellow and white flag, with the seal in the middle of the white stripe, became the official flag of Vatican City.