An increasingly popular feature on my guitars (and others) is the armrest. Armrests serve two functions – firstly they improve playing comfort for the player, by providing a smoothly curved surface for the right arm to rest on, rather than the edge of the guitar, which is usually only very slightly rounded or beveled. The second function is to help keep the player’s arm off the vibrating area of the soundboard. Many players (myself included) don’t find this to be a problem, but it does depend on the design of the guitar, and on how the player holds the guitar.
It is possible to buy aftermarket armrests which attach to the guitar, usually using suction cups. I don’t recommend these for a number of reasons. Firstly, they tend to be quite bulky, and often are not sufficiently long to allow good movement of the players right arm, which is essential if you want to achieve a range of tones from tasto to ponticello. It can also be difficult to get them to attach well to some finishes, particularly those you normally find on hand-built instruments – i.e. French polish or oil finishes.
I offer two types of armrest with my guitars, one which is permanently glued to the edge of the top of the guitar (pictured), and the other which is held by rare earth magnets, and can be removed when required. The latter is slightly wider as its edge extends slightly beyond the outline of the guitar, but both are long enough to allow a good range of movement of the arm, and also slim enough to look elegant and unobtrusive.