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The Pisan Cross is the symbol of Pisa.
The Pisan cross is technically described as with keys (or paws) and three spheres on every arm. Its symbolism is unknown; the cross itself may represent Christ, and the twelve spheres his Apostles.
Although the symbol of the Pisan cross dates to 1017, the oldest surviving representation is in the city walls, built in 1156 by counselor Cocco Griffi. The city flag, red with the white cross on it, was officially recognized by Pope Callixtus II. In fact, originally the flag of Pisa was simply vermillion red, being derived from the Imperial Rome flag.
Nowadays the cross is still the symbol of the city of Pisa. A red shield with the cross on it is the symbol of the Comune of Pisa. It appears in the ensign of the Italian Navy along with the emblems of the other medieval Maritime Republics: Venice, Genoa and Amalfi.