Shoulder rests are an optional component of the viola, sitting between your shoulder the bottom of the viola. They provide support, helping to hold up the viola so that your extended left arm does not have to hold up weight in addition to moving to sound notes. Despite being optional, most players use one as the alternative is to awkwardly tilt your head onto the chin rest to provide the needed support to hold up the viola.
There are several options for shoulder rests and each has varying levels of padding, foldability, and rigidity. As with most other components of the viola, the shoulder rest must feel comfortable to you or else you will be miserable while playing. The viola shoulder rest you choose is all about personal preference.
All shoulder rests contain some sort of padding. While some shoulder rests, like the AcoustaGrip, are all padding, most viola shoulder rests contain a thin layer of padding that covers a rigid plastic or wooden frame.
Most viola shoulder rests are made of a frame out of either plastic, wood, carbon fiber, or aluminum. This frame provides the brace between the shoulder and the bottom of the viola, providing enough elevation to avoid straining the player’s neck. The bottom of the frame is covered with padding to cushion the shoulder and the top is usually covered in some sort of rubber to provide friction between the shoulder rest and the viola. Additionally, some viola shoulder rests can actually fold into a more compact shape to fit in viola cases.