The bridge of a
bowed string instrument is a meticulously
carved piece of
wood that sits vertically on the instrument's top (or front, depending on your view). The
strings are strung over the bridge, and the
vibrations produced by the strings are transmitted into the
instrument, thereby making the whole instrument vibrate.
The bridge is not fastened to the instrument, but held in place by the tension of the strings. The placement of the bridge is determined by the f-holes of the instrument. (The f-holes are f-shaped openings in the top of the instrument, allowing air to move out of the instrument, therefore producing sound). The bridge should be placed directly between the f-holes to give the best distribution of the vibrations. Moving the bridge is not something to take lightly.
On the double bass, adjustable bridges is becoming increasingly common. This involves inserting a metal screw into the "legs" of the bridge. The height of the bridge can then be adjusted depending on playing styles, humidity etc. This can be quite handy, but most people believe it could have a slightly detrimental effect to the sound.
When you're playing with the bow, a special technique exists for playing very close to the bridge. This is called sul ponticello. It is also possible to draw the bow directly on the bridge. This produces an extremely soft breath-like sound, barely audible at distances but nevertheless used occasionally by eager composers.
If amplification of a bowed string instrument is wanted, one of the most common techniques is to put special microphones onto the bridge. A number of different types exist. Many double bassists use this kind of amplification instead of placing a microphone in front of the instrument.