Shoulder rest

The shoulder rest is an accessory that can be found on violins and violas. The shoulder rest appeared around the middle part of the 20th century. It may be made of wood, aluminium, carbon fiber or plastic. Usually, the shoulder rest attaches to the edge of the back of the violin with "feet" padded with rubber tubing or made of soft plastic. The goal of a shoulder rest is to allow a more comfortable attitude while playing by adding height to the shoulder and preventing the instrument from slipping. A shoulder rest generally follows the curve of the shoulder; some shoulder rests are bendable, others are made of sponge-like material, and a few have an extension that hooks further over the shoulder for stability. It is a relatively recent invention, and though it is quite commonly used among modern violinists and violists, it is not universally used. Depending on body type and style of clothes, some musicians prefer no more than a thin sponge, a cloth or sometimes nothing at all under the instrument.
Function.
A shoulder rest, whatever its design, is an accessory to facilitate holding the violin. This can lighten the task of the left hand, enabling smoother and lighter shifting technique. All rests must make a compromise between violin support and freedom (or flexibility in holding the violin). There are a number of soft rests (generally anatomy friendly) and harder or rigid rests on the market.
The shoulder rest is an invention of the middle part of the 20th century. Prior to its invention, violinists and violists employed a number of strategies to hold their instruments: the violin in particular was often depicted in the 16th and 17th centuries being held on or below the collarbone, though this does restrict shifting somewhat. Leopold Mozart is portrayed holding his violin quite low on his shoulder. Large violas were typically played with the aid of straps around the neck, and violins could be attached to the players' cravat. 
There are two common approaches to playing without a shoulder rest. The first is to hold the violin horizontal with the support of the left hand at all times. This usually decreases the mobility of the thumb during downshifts and requires the thumb never to leave the neck of the violin in order to support the instrument. The second approach is to support the instrument almost fully at the neck-shoulder region, often requiring the use of some form of padding beside the collar bone to bring the violin closer to the jaw and to hold the instrument in place. Some violinists may even raise their shoulder to achieve this, particularly when shifting positions and when using vibrato. Problems frequently surface while doing this, such as constant muscle tension which obstructs fluidity of technique, and may lead to injury in the long term. These problems can also arise from incorrect usage of shoulder rests.
Some instruments have been marked due to the friction on the wood and added tension to the instrument the shoulder rest may bring according to each different player strength. There are shoulder rest that do not damage the violin like: AcoustaGrip, Acoustifoam and ZaretUse.