Musical tuning
In music, there are two common meanings for tuning: Tuning practice. Tuning is the process of adjusting the pitch of one or many tones from musical instruments to establish typical intervals between these tones. Tuning is usually based on a fixed reference, such as A = 440 Hz. The term ""out of tune"" refers to a pitch/tone that is either too high (sharp) or too low (flat) in relation to a given reference pitch. While an instrument might be in tune relative to its own range of notes, it may not be considered 'in tune' if it  does not match the chosen reference pitch. Some instruments become 'out of tune' with temperature, humidity, damage, or simply time, and must be readjusted or repaired. Different methods of sound production require different methods of adjustment: The sounds of some instruments, such as cymbals, are "inharmonic" and have irregular overtones not conforming to the harmonic series. Tuning may be done aurally by sounding two pitches and adjusting one of them to match or relate to the other. A tuning fork or electronic tuning device may be used as a reference pitch, though in ensemble rehearsals often a piano is used  (as its pitch cannot be adjusted for each performance). Symphony orchestras and concert bands usually tune to an A440 or a B♭, respectively, provided by the principal oboist or clarinetist, who tune to the keyboard if part of the performance. When only strings are used, then the principal string (violinist) typically has sounded the tuning pitch, but some orchestras have used an electronic tone machine for tuning. Tuning can also be done through a prior recording; this method uses simultaneous audio. Interference beats are used to objectively measure the accuracy of tuning. As the two pitches approach a  harmonic relationship, the frequency of beating decreases. When tuning a unison or octave it is desired to reduce the beating frequency until it cannot be detected. For other intervals, this is dependent on the tuning system being used. Harmonics may be used to facilitate tuning of strings that are not themselves tuned to the unison. For example, lightly touching the highest string of a cello at the middle (at a node) while bowing produces the same pitch as doing the same a third of the way down its second-highest string. The resulting unison is more easily and quickly  judged than the quality of the perfect fifth between the fundamentals of the two strings. Open strings. In music, the term open string refers to the fundamental note of the unstopped, full string. The strings of a guitar are normally tuned to fourths (excepting the G and B strings in standard tuning, which are tuned to a third), as are the strings of the bass guitar and double bass. Violin, viola, and cello strings are tuned to fifths. However, non-standard tunings (called scordatura) exist to change the sound of the instrument or create other playing options. To tune an  instrument, often only one reference pitch is given. This reference is used to tune one string, to which the other strings are tuned in the desired intervals. On a guitar, often the lowest string is tuned to an E. From this, each successive string can be tuned by fingering the fifth fret of an already tuned string and comparing it with the next higher string played open. This works with the exception of the G string, which must be stopped at the fourth fret to sound B against the open B string above. Alternatively, each string can be tuned to  its own reference tone. Note that while the guitar and other modern stringed instruments with fixed frets are tuned in equal temperament, string instruments without frets, such as those of the violin family, are not. The violin, viola, and cello are tuned to beatless just perfect fifths and ensembles such as string quartets and orchestras tend to play in fifths based Pythagorean tuning or to compensate and play in equal temperament, such as when playing with other instruments such as the piano. For example, the cello, which is tuned down from A220, has three more strings (four total) and the  just perfect fifth is about two cents off from the equal tempered perfect fifth, making its lowest string, C−, about six cents more flat than the equal tempered C. This table lists open strings on some common string instruments and their standard tunings from low to high unless otherwise noted. Altered tunings. Violin scordatura was employed in the 17th and 18th centuries by Italian and German composers, namely, Biagio Marini, Antonio Vivaldi, Heinrich Ignaz Franz Biber (who in the "Rosary Sonatas" prescribes a great variety of scordaturas, including crossing the middle strings), Johann Pachelbel and Johann Sebastian Bach, whose "Fifth  Suite For Unaccompanied Cello" calls for the lowering of the A string to G. In Mozart's "Sinfonia Concertante" in E-flat major (K. 364), all the strings of the solo viola are raised one half-step, ostensibly to give the instrument a brighter tone so the solo violin does not overshadow it. Scordatura for the violin was also used in the 19th and 20th centuries in works by Niccolò Paganini, Robert Schumann, Camille Saint-Saëns and Béla Bartók. In Saint-Saëns' "Danse Macabre", the high string of the violin is lower half a tone to the E so as  to have the most accented note of the main theme sound on an open string. In Bartók's "Contrasts", the violin is tuned G-D-A-E to facilitate the playing of tritones on open strings. American folk violinists of the Appalachians and Ozarks often employ alternate tunings for dance songs and ballads. The most commonly used tuning is A-E-A-E. Likewise banjo players in this tradition use many tunings to play melody in different keys. A common alternative banjo tuning for playing in D is A-D-A-D-E. Many Folk guitar players also used different  tunings from standard, such as D-A-D-G-A-D, which is very popular for Irish music. A musical instrument that has had its pitch deliberately lowered during tuning is said to be "down-tuned" or "tuned down". Common examples include the electric guitar and electric bass in contemporary heavy metal music, whereby one or more strings are often tuned lower than concert pitch. This is not to be confused with electronically changing the fundamental frequency, which is referred to as pitch shifting. Tuning of unpitched percussion instruments. Many percussion instruments are "tuned" by the player, including pitched percussion  instruments such as timpani and tabla, and unpitched percussion instruments such as the snare drum. Tuning pitched percussion follows the same patterns as tuning any other instrument, but tuning unpitched percussion does not produce a specific pitch. For this reason and others, the traditional terms "tuned percussion" and "untuned percussion" are avoided in recent organology. Tuning systems. A "tuning system" is the system used to define which tones, or pitches, to use when playing music. In other words, it is the choice of number and spacing of frequency values used. Due to the psychoacoustic interaction of tones and timbres, various  tone combinations sound more or less "natural" in combination with various timbres. For example, using harmonic timbres: More complex musical effects can be created through other relationships. The creation of a tuning system is complicated because musicians want to make music with more than just a few differing tones. As the number of tones is increased, conflicts arise in how each tone combines with every other. Finding a successful combination of tunings has been the cause of debate, and has led to the creation of many different tuning systems across the world. Each tuning system has its own characteristics, strengths  and weaknesses. Systems for the twelve-note chromatic scale. It is impossible to tune the twelve-note chromatic scale so that all intervals are pure. For instance, three pure major thirds stack up to 125/64, which at 1159 cents is nearly a quarter tone away from the octave (1200 cents). So there is no way to have both the octave and the major third in just intonation for all the intervals in the same twelve-tone system. Similar issues arise with the fifth 3/2, and the minor third 6/5 or any other choice of harmonic-series based pure intervals. Many  different compromise methods are used to deal with this, each with its own characteristics, and advantages and disadvantages. The main ones are: Tuning systems that are not produced with exclusively just intervals are usually referred to as "temperaments".