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In musical tuning, a temperament is a system of tuning which slightly compromises the pure intervals of just intonation in order to meet other requirements of the system. Most instruments in modern Western music are tuned in the equal temperament system. Temperament is especially important for keyboard instruments, which typically allow a player to play only the pitches assigned to the various keys, and lack any way to alter pitch of a note in performance. Historically, the use of just intonation, pythagorean tuning and meantone temperament meant that such instruments could sound "in tune" in one key, or some keys, but would then have more dissonance in other keys.
The development of well temperament allowed fixed-pitch instruments to play reasonably well in all of the keys. The famous "Well-Tempered Clavier" by Johann Sebastian Bach takes full advantage of this breakthrough, with pieces written for all 24 major and minor keys. However, while unpleasant intervals (such as the wolf interval) were avoided, the sizes of intervals were still not consistent between keys, and so each key still had its own character. This variation led in the 18th century to an increase in the use of equal temperament, in which the frequency ratio between each pair of adjacent notes on the keyboard was made equal, allowing music to be transposed between keys without changing the relationship between notes.
What temperament is 
In just intonation, every interval between two pitches corresponds to a whole number ratio between their frequencies. For instance, 660 Hz / 440 Hz constitutes a fifth, and 880 Hz / 440 Hz an octave. Such intervals (termed "just") have a stability, or purity to their sound, when played simultaneously (assuming they are played using timbres with harmonic partials). If one of those pitches is adjusted slightly to deviate from the just interval, a trained ear can detect this change by the presence of beats, which are explained below.
Acoustic physics 
When a musical instrument with harmonic overtones is played, the ear hears a composite waveform that includes a fundamental frequency (e.g., 440 Hz) and those overtones (880 Hz, 1760 Hz, etc.) – a series of just intervals. The waveform of such a tone (as pictured on an oscilloscope) is characterized by a shape that is complex compared to a simple (sine) waveform, but which remains periodic. When two tones depart from exact integer ratios, the shape waveform becomes erratic – a phenomenon which may be described as destabilization. As the composite waveform becomes more erratic, the consonance of the interval also changes.
Temperament in music 
Tempering an interval involves the deliberate use of such minor adjustments (accepting the related destabilization) to enable musical possibilities that are impractical using just intonation. The most widely known example of this is the use of equal temperament to address problems of older temperaments, allowing for consistent tuning of keyboard and fretted instruments and enabling musical composition in, and modulation among, the various keys.
Meantone temperament 
Before Meantone temperament became widely used in the Renaissance, the most commonly used tuning system was Pythagorean tuning. Pythagorean tuning was a system of just intonation which tuned every note in a scale from a progression of pure perfect fifths. This was quite suitable for much of the harmonic practice until then (See: Quartal harmony), but in the Renaissance, musicians wished to make much more use of Tertian harmony. The major third of Pythagorean tuning differed from a just major third by an amount known as Syntonic comma, which musicians of the time found annoying.
Their solution, laid out by Pietro Aron in the early 16th century, and referred to as meantone temperament (or quarter-comma meantone temperament), was to temper the interval of a perfect fifth slightly narrower than in just intonation, and then proceed much like Pythagorean tuning, but using this tempered fifth instead of the just one. With the correct amount of tempering, the Syntonic comma is removed from its major thirds, making them just. This compromise, however, leaves all fifths in this tuning system with a slight beating. However, because a sequence of four fifths makes up one third, this beating effect on the fifths is only one quarter as strong as the beating effect on the thirds of Pythagorean tuning, which is why it was considered a very acceptable compromise by Renaissance musicians.
Pythagorean tuning also had a second problem, which Meantone temperament does not solve, which is the problem of modulation (see below), which is restricted because both have a broken circle of fifths. A series of 12 just fifths as in Pythagorean tuning does not return to the original pitch, but rather differs by a Pythagorean comma, which makes that tonal area of the system more or less unusable. In meantone temperament, this effect is even more pronounced (the fifth over the break in the circle is known as the Wolf interval, as its intense beating was likened to a "howling".) 53 equal temperament provides a solution for the Pythagorean tuning, and 31 equal temperament for the Meantone.
Well temperament and Equal temperament 
Just intonation has the problem that it cannot modulate to a different key (a very common means of expression throughout the common practice period of music) without discarding many of the tones used in the previous key, thus for every key to which the musician wishes to modulate, the instrument must provide a few more strings, frets, or holes for him or her to use. When building an instrument, this can be very impractical.
Well temperament is the name given to a variety of different systems of temperament that were employed to solve this problem, in which some keys are more in tune than others, but all can be used. This phenomenon gives rise to infinite shades of key-colors, which are lost in the modern standard version: 12 tone equal temperament (12-TET). Unlike meantone temperament, which alters the fifth to temper out the Syntonic comma, 12-TET tempers out the Pythagorean comma, thus creating a cycle of fifths that repeats itself exactly after 12 steps. This allowed the intervals of Tertian harmony, thirds and fifths, to be fairly close to their just counterparts (the fifths almost imperceptibly beating, the thirds a little milder than the syntonic beating of Pythagorean tuning), while permitting the freedom to modulate to any key and by various means (e.g. common-tone and enharmonic modulation, see modulation). This freedom of modulation also allowed substantial use of more distant harmonic relationships, such as the Neapolitan chord, which became very important to Romantic composers in the 19th century.
Frequently used equal temperament scales 
- 12 tone equal temperament
- 15 tone equal temperament
- 17 tone equal temperament
- 19 tone equal temperament
- 22 tone equal temperament
- 24 tone equal temperament
- 31 tone equal temperament
- 34 tone equal temperament
- 41 tone equal temperament
- 53 tone equal temperament
- 72 tone equal temperament
See also 
- Piano tuning
- Regular temperament
- List of meantone intervals
- Whole-tone scale
- Pythagorean interval
- Mathematics of musical scales
- Schismatic temperament
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- Jorgensen, Owen. Tuning: Containing the Perfection of Eighteenth-Century Temperament; The Lost Art of Nineteenth Century Temperament; and The Science of Equal Temperament. Michigan State University Press, 1991. ISBN 0-87013-290-3
- Miller, Willis G. The Effects of Non-Equal Temperament on Chopin's Mazurkas. PhD diss., University of Houston, October 2001.
- Pressler, James. The Temperamental Mr. Purcell. Frog Music Press, UPC 883629638829
- Steblin, Rita. A History of Key Characteristics in the 18th and Early 19th Centuries. UMI Research Press, Ann Arbor, 1983.
- Music: a Mathematical Offering (mathematical perspective with two chapters on temperament) by Dave Benson
- Tuning And Temperament A Historical Survey (1951) by J. Murray Barbour
- Essay on Musical Temperament (part 2) by Prof. Fisher (Yale College)
- "Temperament" from A supplement to Mr. Chambers's cyclopædia (1753)
- Theory and practice of just intonation (1850) by Thomas Perronet Thompson
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- An essay on temperament (1832) by J. Jousse
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- Piano Tuning: A Simple and Accurate Method for Amateurs by Jerry Cree Fischer
- The organ viewed from within : a practical handbook on the mechanism of the organ, with a chapter on tuning by John Broadhouse
- Construction, Tuning and Care of the Piano-forte (1887) by Edward Quincy Norton
- Regulation and Repair of Piano and Player Mechanism, Together with Tuning as Science and Art (1909) by William Braid White
- Modern piano tuning and allied arts (1917) by William Braid White (1878-1959)
- Biddle, Horace Peters (1867). The Musical Scale. Oliver Ditson & Company.