The flag of Vatican City was adopted on June 7, 1929, the year Pope Pius XI signed the Lateran Treaty with Italy, creating a new independent state governed by the Holy See. The Vatican flag is modeled on the flag of the earlier Papal States.

The flag consists of two vertical bands, one of gold (hoist side) and one of white with the crossed keys of Saint Peter and the Papal Tiara centred in the white band. The crossed keys consist of a golden and a silver key, in which the silver key is placed in the dexter position. The flag is one of only two square country flags in the world, the other being the flag of Switzerland.

The flag is flown or displayed worldwide in Roman Catholic churches and institutions, usually alongside the national flag of where the church or institution is located.

The crossed keys was the Papal States coat of arms showing how important the Papal States were to Vatican City. Wikipedia