Together with the mainplate and the pillars, the bridges are part of what is called the movement cage, not to be confused with the tourbillon cage. They are essential elements to maintain the mobiles fixed on the mainplate.
They can be four or five per movement and sometimes less. German watchmaking is for example known for its three-quarter bridges where a single bridge covers three quarters of the movement, so there are fewer bridges used on this type of construction.
Bridges are often machined in brass, sometimes in nickel silver, sometimes on old steel pieces for anchor bridges for example, and in rare cases in gold.
