The Baptistery, dedicated to St. John the Baptist, stands opposite the west end of the Duomo. The round Romanesque building was begun in the mid 12th century: 1153 Mense August fundata fuit haec ("In the month of August 1153 was set up here..."). It was built in Romanesque style by an architect known as Diotisalvi ("God Save You"), who worked also in the church of the Holy Sepulchre in the city. His name is mentioned on a pillar inside, as Diotosalvi magister. the construction was not, however, finished until the 14th century, when the loggia, the top storey and the dome were added in Gothic style by Nicola Pisano and Giovanni Pisano.

It is the largest baptistery in Italy, with a circumference measuring 107.25 m. Taking into account the statue of St. John the Baptist (attributed to Turino di Sano) atop the dome, it is even a few centimetres taller than the Leaning Tower.

The portal, facing the façade of the cathedral, is flanked by two classical columns, while the inner jambs are executed in the Byzantine style. The lintel is divided into two tiers, the lower one depicting several episodes in the life of St. John the Baptist, and the upper one showing Christ between the Madonna and St. John the Baptist, flanked by angels and the evangelists.

The immensity of the interior is overwhelming, but it is surprisingly plain and lacking in decoration. It has notable acoustics also.

The octagonal baptismal font at the centre dates from 1246 and was made by Guido Bigarelli da Como. The bronze sculpture of St. John the Baptist at the centre of the font is a remarkable work by Italo Griselli.

The pulpit was sculpted between 1255-1260 by Nicola Pisano, father of Giovanni Pisano, the artist who produced the pulpit in the Duomo. The scenes on the pulpit, and especially the classical form of the naked Hercules, show at best Nicola Pisano's abilities as the most important precursor of Italian renaissance sculpture by reinstating antique representations. Therefore, surveys of the Italian Renaissance usually begin with the year 1260, the year that Nicola Pisano dated this pulpit.

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