The bell-tower is the resonance chamber and together with the bells constitutes a single instrument with a unique and unrepeatable sonority

The tower acts as a resonance chamber and modifies the sounds according to its internal structure, layout and the number, size and kind of bells, so each bell-tower has its own, unique and unrepeatable sonority. Therefore, changes to the structure of the belfry directly or indirectly affect the sound of the bells. This means we cannot look after the bells without protecting the bell-tower, and we cannot modify the tower without affecting the bells.

Bells and bell-tower are a unity made up of different but coordinated elements. They make a complex musical instrument which has managed to evolve and diversify in different regions and countries, adopting their own languages by different ways of striking the bells —with hammers and with clappers— of ringing —tolling or turning over— of mounting in the tower —inside or hanging in open compartments in the walls as is usual in Spain—.

In Bolonia cathedral the bells are inside the tower, have clappers and are tolled below and flipped over above from a platform which allows the ringers to flip them over using levers. In Utrecht cathedral, the bells are inside and above and connected to the ringers bellow by long ropes.

All this means that within a structural uniformity there is a wide variety of architectures which give rise to unique and individualised instruments, due to their appearance and their voices.