Bell tower is an architectural structure that is ordinarily located above the western end of a church, where the bells are placed. Depending on their shape, bell towers have different names. Those that diminish in size towards the top like a cone with a circular base are called needles. Those the base of which is rectangular, pentagonal or hexagonal and the diameter of which also diminish in size as they approach their summit are called pyramids. The former and the latter have apertures: these apertures are covered with screens, which are nothing other than inclined frames, covered with slate, which serve to send the sound of the bells downwards.

A basic bell tower begins on ground level and grows without interruption to the height of the church, like those of Saint Eustache, Saint Sulpice, etc. Sometimes these towers, most usually square in design, are capped with needle- or arrow-shaped structures, like the portal of Rheims Cathedral, or by a small attic, like that of Saint Jean en greve, or finally by a flat top, like Notre Dame of Paris.

Masius, in his treaty on bells, notes that the bell tower of Pisa is the most unique and remarkable in the world. It leans, he says, towards one side, and always appears to be falling: he assures us, however, that this extraordinary appearance is not the result of an earthquake, as some imagine. It is rather the intention of the architect who built it. This is also evidenced in the floorboards, the doors and the windows, which all are arranged according to the same angle despite the inclination of the tower.