All advancing string players need to know how to adjust their bridges. The youngest of players should probably put off learning this task off until they are old enough to accomplish it with a reasonable degree of success, which means that it incumbent upon parents of young players to learn the procedure as well. Archtop instruments (mandolins, archtop guitars, violins, violas, cellos, double basses, etc.) are unique in design because their bridges are not fastened down. The float on the top and are held in place only by the tension of the strings. When a string vibrates on one of these instruments the bridge moves back and forth in a semi-piston fashion – enhancing the volume and tone of the instrument. Also, if the bridges were glued down it would make it more difficult to work on the instruments and more difficult to adjust or replace the bridge.
Adjusting the height and shape of a bridge (cutting the top of the bridge) is definately a job best left for a professional luthier and should not be attempted by the musician (unless they are commited to investing a fair amount of time, expense and dedication to developing the skill. I don’t recommend it). What we are discussing here is adjusting the placement of the bridge on the top. If a bridge is bumped, even a little, it can maladjust the position of the bridge and compromise the sound, intonation and playability of the instrument. This is very common. Also, whenever new strings are put on a violin they will tend to pull the bridge over on an angle to the top as they are constantly stretching and being re-tuned. If the bridge is left in a leaning position it can warp the bridge. If the bridge leans too far over it will fall with a gunshot like crash and scare the begebbies out of you. Most of the time the violin or cello with not be damaged but it is not worth the heart palpatations that you will experience.
With a violin or viola, place the instrument on your lap with the neck pointing away from you or set it on a well padded table. Take the bridge between your fingers and thumbs while grasping the violin between your lower palms. You want a good grip on the violin so that it doesn’t move when you adjust the bridge. There are holes in the top of the violin in the shape of a French F. There are notches on these F holes and the bridge should be centered between the notches. Also, the strings should be centered on the fingerboard with about the same distance between the strings and the edges of the finger board on either sides. Finally, examine where the bridge meets the top of the instrument and make sure that the bridge is straight up and down without gaps between the feet and the top of the violin. Take your time and make sure that everything looks right. This is not rocket science but don’t be in a rush. The bridge needs to be centered on the notches, the strings need to be centered on the fingerboard and the bridge needs to be straight up and down. If it looks a little off it probably is so try again. I occasionally see bridges that have accidentally been turned around. This will be obvious because the biggest string will be way too low on the fingerboard and the littlest string will be way too high. On a full size violin the E will be about 1/8 from the fingerboard at the end and the G string about 3/16ths.
The cello is very similar but obviously bigger than a violin or viola. I like to put it on a couch with the bottom resting against the arm of the couch (make sure that the couch is large enough to hold the cello without the neck hanging over the other arm and endangering the neck joint). You can also use a bed or mattress. Again, use two hands on the bridge and don’t rush so you don’t drop the bridge over on the top.
If the bridge has fallen or if you are having trouble moving the bridge it isn’t a bad idea to lower the tension of the strings but note that when you lower the string tension and then set the bridge up – the bridge will tend to pull over as you raise the tension back up to proper pitch so you have to monitor and adjust the straightness of the bridge as you go. Here is a good video that you might find useful: