The Leaning Tower of Pisa is the campanile, or freestanding bell tower, of the cathedral of the Italian city of Pisa, known worldwide for its nearly four-degree lean, the result of an unstable foundation. The tower is situated behind the Pisa Cathedral and is the third-oldest structure in the city's Cathedral Square, after the cathedral and the Pisa Baptistry.

Construction of the tower occurred in three stages over 199 years, from 1173 to 1372. The bell-chamber was finally added in 1372. It was built by Tommaso di Andrea Pisano, who succeeded in harmonizing the Gothic elements of the belfry with the Romanesque style of the tower. There are seven bells, one for each note of the musical major scale.

The seven bells are:

1st bell: L'Assunta, cast in 1654 by Giovanni Pietro Orlandi, weight 3,620 kg (7,981 lb), 2nd bell: Il Crocifisso, cast in 1572 by Vincenzo Possenti, weight 2,462 kg (5,428 lb), 3rd bell: San Ranieri, cast in 1719–1721 by Giovanni Andrea Moreni, weight 1,448 kg (3,192 lb), 4th bell: La Terza (1st small one), cast in 1473, weight 300 kg (661 lb), 5th bell: La Pasquereccia or La Giustizia, cast in 1262 by Lotteringo, weight 1,014 kg (2,235 lb), 6th bell: Il Vespruccio (2nd small one), cast in the 14th century and again in 1501 by Nicola di Jacopo, weight 1,000 kg (2,205 lb) and the 7th bell: Dal Pozzo, cast in 1606 and again in 2004, weight 652 kg (1,437 lb).

More Info: en.wikipedia.org