The arms show the crossed keys of St.Peter, the patron saint of the city. 

When the keys were first used on the arms of the city is not clear; they are known since around 1300 as arms of the Counts of Regensburg and since 1320 as a symbol of the city. On seals they first appear in a small shield on a seal dating from the end of the 14^thcentury. The main seal of the city, which dates from the end of the 12^thcentury, shows St. Peter sitting on a throne, holding a key. Smaller seals from the 13th and 14^th century show only the bust of St. Peter, wearing a tiara, or papal crown. All small seals since 1519 show only the arms with the crossed keys. 

Similarly, the city uses since the middle of the 14^th century only the keys as its symbol, either in or without a shield.