may be plated or plain. Bluegrass players tend to use square necks, while blues players tend to prefer round necks. Square-necked guitars give a slightly greater variety of possible tunings, while round-necked guitars give a much greater variety of playing positions. Single resonator instruments can have round sound holes with screens, or round sound holes without screens, which many players used to remove to improve the bass response. They can also have f-holes, often with gauze screens that are also sometimes removed but have an important function in strengthening the belly particularly if the body is of wood. An enormous number of combinations are possible, most can be found either on old or new instruments or both, and many styles of music can be played on any resonator guitar. Electric resonators Though the original aim of the resonator was increased volume, some modern instruments incorporate electric pickups and related technology. Many modern makers produce instruments with one of a variety of pickup types—and some players retrofit pickups to non-electric instruments. Most commonly, resonator guitars use piezoelectric pickups (contact type transducers) placed under the bridge or elsewhere on the instrument, or use specialized microphones placed inside the instrument or directly in front of the cone to preserve the resonator's distinctive tone. However, all acoustic and semi-acoustic styles are very sensitive to audio feedback, making the design and placement of these pickups extremely critical and specialized. Some modern models are manufactured with both piezoelectric and magnetic pickups. In addition, some piezoelectric styles are active pickups, in that they incorporate a preamplifier that increases the output of the pickup to match modern amplifier inputs. More recently, solid body electric resonator guitars have appeared. These instruments incorporate one or more magnetic pickups, and are played via amplification. Other resonator instruments As well as resonator guitars, resonators have been used on: Basses, available from Regal Ukuleles, (see Resonator ukulele) produced by National and Dobro 1928-1940 Banjos Tenor guitars Mandolins and mandolas Mountain/Appalachian dulcimers Viola guitars Brands Historic brands of resonator guitar still in use today include National, Dobro, and Regal. None of these brands are still owned by their original companies. Each returned after one or more long breaks in production: The National name is now used by National Reso-Phonic Guitars, a company founded in 1987 and unconnected to the original National, specializes in reproductions of historic instruments of all brands, not just National pattern