The Pisan Cross is the symbol of Pisa

Pisa

Pisa is a city in Tuscany, Central Italy, on the right bank of the mouth of the River Arno on the Tyrrhenian Sea. It is the capital city of the Province of Pisa...

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It was the coat of arms of the people of Pisa: the symbol of the cross was granted, in fact, to pisans by Pope Benedict VIII

Pope Benedict VIII

Pope Benedict VIII , born Theophylactus, Pope from 1012 to 1024, of the noble family of the counts of Tusculum , descended from Theophylact, Count of Tusculum like his predecessor Pope Benedict VI .Benedict VIII was opposed by an antipope, Gregory...

 to fight saracen

Saracen

Saracen was a term used by the ancient Romans to refer to a people who lived in desert areas in and around the Roman province of Arabia, and who were distinguished from Arabs. In Europe during the Middle Ages the term was expanded to include Arabs, and then all who professed the religion of Islam...

s in Sardinia

Sardinia

Sardinia is the second-largest island in the Mediterranean Sea . It is an autonomous region of Italy, and the nearest land masses are the French island of Corsica, the Italian Peninsula, Sicily, Tunisia and the Spanish Balearic Islands.The name Sardinia is from the pre-Roman noun *sard[],...

 in 1017.

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

Pope Callixtus II

Pope Calixtus II , born Guy de Vienne, the fourth son of William I, Count of Burgundy , was elected Pope on February 1, 1119, after the death of Pope Gelasius II . His pontificate was shaped by the Investiture Controversy, which he was able to settle through the Concordat of Worms...

. 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

Comune

In Italy, the comune is the basic administrative division, and may be properly approximated in casual speech by the English word township or municipality.-Importance and function:...

 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.