A chinrest is a shaped piece of wood (or plastic) attached to the body of a violin or a viola to aid in the positioning of the player's jaw or chin on the instrument. The chinrest may be made of ebony, rosewood, boxwood, or plastic. It was invented by Louis Spohr in the early 19th century in response to increasingly difficult repertoire which demanded freer left hand techniques than had previously been used. After being promoted by prominent violinists of the day, such as Pierre Baillot and Giovanni Battista Viotti, it gained quick acceptance among most violists & violinists and is today considered a standard part of the viola and violin.
Chinrest attachment 
The chinrest is attached to the instrument by one or two metal clamps that hook over the edge of the back. Two bottlescrews comprise the main parts of the clamp or clamps and provide slight tension to hold the chinrest in place. When attachment is by a single clamp, the two screws are joined by a metal bar that conforms to the back edge of the instrument, whereas "Hill-style" clamps are used in a pair, each clamp having its own foot and its own screw, with no bar joining them. The clamps and the chinrest itself are usually padded with cork or felt where they contact the instrument to minimize damage to the wood.
The two screws may both be placed on the shoulder side of the instrument, or one may be placed on each side of the endpin so that the chinrest straddles the tailgut. The latter may be considered a safer installation, as the clamps get support from the bottom block, and there is less chance that overtightening will cause damage.
A special wrench may be used for easy adjustment of the clamp tension without damaging the rib of the violin. The clamps of some modern composite chinrests are adjusted with a small Phillips screwdriver or with a hex key.
Spohr's original design called for a small block of wood to be centered over the tailpiece, but soon evolved. Today, there are at least 50 different types of chinrests available.
Some players prefer a chinrest with the cup centered over the tailpiece. The "Flesch" chinrest is of this type.
As described in the preceding section, some chinrests attach to the left of the endpin.
Chinrests are available in different heights and shapes. Violin and viola pedagogue Susan Kempter advocates having a luthier customize the chinrest by shaping it to fit the player's jaw properly and either raising or reducing the height until it fits the player's neck height as well.
Other considerations 
The chinrest should be placed so it does not touch or buzz against the tailpiece or belly of the instrument.
Pressure from the chinrest against the player's skin can result in a common irritation known as "fiddler's neck" or "plague". It can also be caused by bacteria or fungus living on the wood or by an allergic reaction to the metals used in the chinrest. Some players prefer to use a cloth, such as a handkerchief, to cover the chinrest, to avoid this irritation and make playing more comfortable. Several types of padded fabric slipcovers are also commercially available. They may not only cover the chinrest cup, but also extend to provide a barrier between the metal clamp hardware and the skin—beneficial if the hardware is plated with nickel and the player is allergic to the metal. Hypo-allergenic chinrests (with plastic or titanium fittings) are also available.
Vociferous debate may be elicited in some circles by suggesting that a violin sounds better without a chinrest. It has been suggested that Paganini played without a chinrest, though no proof exists. A few professionals today (notably Regina Carter) do not use a chinrest on their modern instruments; many who play Baroque instruments also avoid one.
See also 
- Nemet, Mary (August/September). "Choosing a chinrest". Strings (121)
- Ford, Charles (1979). Making Musical Instruments: Strings and Keyboard. New York: Pantheon Books, a division of Random House, Inc. p. 97. ISBN 0-394-73561-7.
- Kempter, Susan (2003). How Muscles Learn: Teaching the Violin with the Body in Mind. 15800 N.W. 48th Avenue, Miami, FL, 33014: Summy-Birchard, Inc., distributed by Warner Bros. Publications. p. 23.