The campanile (bell tower), commonly known as the Leaning Tower of Pisa, is located behind the cathedral. The last of the three major buildings on the piazza to be built, construction of the bell tower began in 1173 and took place in three stages over the course of 177 years, with the bell-chamber only added in 1372. Five years after construction began, when the building had reached the third floor level, the weak subsoil and poor foundation led to the building sinking on its south side. The building was left for a century, which allowed the subsoil to stabilise itself and prevented the building from collapsing. In 1272, to adjust the lean of the building, when construction resumed, the upper floors were built with one side taller than the other. The seventh and final floor was added in 1319. By the time the building was completed, the lean was approximately 1 degree, or 80 cm (2.5 feet) from vertical. At its greatest, measured prior to 1990, the lean measured approximately 5.5 degrees. As at 2010, the lean was reduced to approximately 4 degrees.