Because they were not used until fairly recently and also because they may affect the sound, there are still some teachers who recommend against using them at all.
Certainly, there are some violinists who do fine without a shoulder rest.
Often, these people have short necks.
For example, David Oistrakh, one of my favorite recording artists, fits this mold.
For the rest of us, a shoulder rest can make playing the violin much more comfortable.
It frees the left hand from having to support the instrument and should allow the violinist to play without raising the left shoulder.
The choice of shoulder rest depends on the unique physiology of each student.
There are two major categories of shoulder rests.
The first are is the group of "soft" rests (such as curved foam pieces, sponges --even a kitchen sponge with a rubber band will do-- and the Play-on-Air, which is formed of a "bladder" filled with air that can be adjusted by varying the amount of air contained in the bladder.
Some teachers prefer rests that are not rigid.
Their logic is that a rigid rest locks the player in one position and can add tension.
However, the sound can be affected by the pressure of the soft pad against the back of the violin.
Often, the contact of the soft rest will muffle or create fuzz in the sound.
The second major category of shoulder rests are composed of the "rigid rests.
" There rigid models are based upon the same principal: feet that attach to the sides of the violin and a curved, rigid, shaped bar that connects the feet.
Each brand of rigid rest is shaped slightly differently, which provides a good variety to fit the many different shapes of violinist's upper body, neck height, collar bone prominence, and slope and width of shoulders.
To help with selection, I have assembled a list of the more popular brands of rigid rests with descriptions of each and tips on which model will best fit a given violinist's unique physiology.
To begin, the Everest is made in America and is relatively cheap.
The padding is thick and the rest seems to fit well for medium to long necks.
One model offers collapsible, folding feet.
The Bonmusica is made of flexible metal with a "hook" that can be molded to fit over the shoulder.
It is a heavy rest composed of aluminum that can affect the sound more than some but can work well for violinists who have not found any of the other rests to be comfortable because of it's high degree of flexibility and ability to curve around the back of the shoulder.
Kun is the original inventor of the rigid rest with feet.
There are several different models.
They tend to fit most people fairly well.
One model has a bar connecting the feet that can tilt towards and away from the player.
Though potentially a very valuable feature, this bar cannot be locked in the chosen ideal position and therefore must be adjusted sometimes several times in a playing session.
The Mach One is very light and made of nice quality solid maple.
Some people find it very comfortable but it is rather short and the padding can seem slippery and is quite thin.
The Wolf is an excellent rest for very tall necks, but can feel very rigid.
Though it appears to have no curve, it can be bent to suit the player.
Individuals with "A-Frame" (highly sloped) shoulders sometimes find this is their only viable option as it is the tallest rest.
The viva la musica rest comes in many colors and adjusts in two planes.
This extra ability to adjust allows it to accommodate certain violinists who have had difficulty finding a comfortable rest.
However, the bar is fairly flat, with little curve, and can rub against the backside of the instrument.
In addition, this rest is composed of plastic with a tendency of breaking sooner than other rests.
In sum, my recommendation for selecting a shoulder rest is to try several and look for a comfortable fit.
In the case of the rigid models with feet, the pad should rest securely against your shoulder and collar bone with little if any gaps between you and the rest.
To adjust the rest on the violin, experiment with different placements for the feet.
In general, the feet under the chin rest fill the space between the shoulder and the back of the violin.
The closer the feet are positioned to the chin rest, the more the violin will tend to rest away from the end of the shoulder and more towards the neck.
The height should be set to fill the space between the jaw and the top of the shoulder.
On the opposite side of the instrument, a placement for the feet closer to the scroll side will position the instrument more toward the center of the chest and away from the shoulder.
More height tends to flatten the instrument, and less height tends to create more slope.
A shoulder rest that is too high will tilt the neck and chin upward, which is often very uncomfortable, along with raising the violin and thus the entire playing posture, which can place more strain on the shoulder as it stretches upward towards a higher violin.
But if the rest is positioned too low, the player must bend the neck excessively to reach the chin rest, which especially deleterious to the health of the neck.
The importance of the shoulder rest can easily be underestimated, and it is often useful to ask your teacher to help you assess fit.