instruments have a compensated nut. This type of nut provides better average theoretical intonation across the instrument, although this improved accuracy may be below the threshold of human ability to hear it and may also be below the threshold of uncontrollable note-to-note intonation variability. The principle given that strings are different thicknesses and have different tensions, the temperament of each fret is not 100% accurate for an equal temperament instrument. This is especially evident on the first few frets of an electric guitar. Many guitar players notice how 'open position' chords (Such as E, A, C, D and G) never sound in tune with each other. A compensated nut aims to correct this, by staggering the starting position of each string according to thickness. While not a complete solution such as a true temperament fretboard, there is a noticeable difference in tuning within chords. Many guitar companies, such as Music Man, and ESP include compensated nuts as standard on most of their instruments, and companies such as Earvana provide retrofittable types. Another type is a locking nut. This nut—usually used in conjunction with a locking vibrato system such as a Floyd Rose or Kahler—clamps the strings against the nut. This improves tuning stability when using the vibrato bar. A drawback however, is that the locking nut must be loosened using an Allen wrench to tune outside the range of the fine tuners on the bridge (if present). The erhu does not use a hard nut to define the vibrating length of the open string, but rather a qiān jin (千斤), a loop of string, or, less commonly, a metal hook. Some guitars have a rolling nut. In this design, made popular by Fender, the strings sit on roller bearings instead of nut slots. The rollers let the string freely slide or roll through the nut. The roller nut helps keep the guitar in tune by preventing the strings from getting stuck in the nut. Keyboard instruments The term "nut" also refers to bridges on certain keyboard instruments. On harpsichords, it designates the non-sounding bridge located near the tuning pins away from the player. On virginals, the term usually designates the bridge on the left side, away from the tuning pins. The term is not always applied consistently. References String instrument construction Musical instrument parts and accessories