Editing Perfect fourth (section)

[[Image:Perfect fourth on C.png|thumb|right|Perfect fourth {{audio|Perfect fourth on C.mid|Play}}]]
{{Infobox Interval| 
       main_interval_name = perfect fourth|
       inverse = [[perfect fifth]]|
       complement = [[perfect fifth]]|
       other_names = diatessaron| 
       abbreviation = P4 |
       semitones = 5 | 
       interval_class = 5 | 
       just_interval = 4:3| 
       cents_equal_temperament = 500|
       cents_24T_equal_temperament = |
       cents_just_intonation = 498
}}

A '''fourth''' is a [[interval (music)|musical interval]] encompassing four [[staff position]]s in the music notation of [[Western culture]], and a '''perfect fourth''' ({{audio|Perfect fourth on C.mid|Play}}) is the fourth spanning five [[semitone]]s (half steps, or half tones). For example, the ascending interval from C to the next F is a perfect fourth, because the note F is the fifth semitone above C, and there are four staff positions between C and F. [[Diminished fourth|Diminished]] and [[Tritone|augmented fourth]]s span the same number of staff positions, but consist of a different number of semitones (four and six, respectively).

The perfect fourth may be derived from the [[Harmonic series (music)|harmonic series]] as the interval between the third and fourth harmonics. The term ''perfect'' identifies this interval as belonging to the group of perfect intervals, so called because they are neither major nor minor.

A perfect fourth in [[just intonation]] corresponds to a pitch ratio of 4:3, or about 498 [[cent (music)|cent]]s ({{audio|Just perfect fourth on C.mid|Play}}), while in [[equal temperament]] a perfect fourth is equal to five semitones, or 500 cents (see [[additive synthesis]]).

Until the late 19th century, the perfect fourth was often called by its Greek name, ''diatessaron''.<ref>{{cite book  |author=William Smith and Samuel Cheetham |title=A Dictionary of Christian Antiquities |year=1875 |publisher=John Murray |location=London |isbn=9780790582290 |url=https://books.google.com/books?id=1LIPFk6oFVkC&q=diatessaron+diapason+diapente+fourth+fifth&pg=PA550}}</ref> Its most common occurrence is between the [[Fifth (chord)|fifth]] and upper [[root (chord)|root]] of all [[Major chord|major]] and [[Minor chord|minor triad]]s and their [[extended chords|extensions]].

An example of a perfect fourth is the beginning of the "Bridal Chorus" from [[Wagner]]'s ''[[Lohengrin (opera)|Lohengrin]]'' ("''Treulich geführt''", the colloquially-titled "[[Bridal Chorus|Here Comes the Bride]]"). Another example is the beginning melody of the [[State Anthem of the Soviet Union]]. Other examples are the first two notes of the [[Christmas carol]] "[[Hark! The Herald Angels Sing]]" and "[[El Cóndor Pasa (song)|El Cóndor Pasa]]", and, for a descending perfect fourth, the second and third notes of "[[O Come All Ye Faithful]]".{{citation needed|date=May 2014}}

The perfect fourth is a perfect interval like the [[unison]], [[octave]], and [[perfect fifth]], and it is a sensory [[consonance and dissonance|consonance]]. In [[common practice period|common practice]] harmony, however, it is considered a stylistic dissonance in certain contexts, namely in two-voice textures and whenever it occurs "above the bass in chords with three or more notes".<ref>{{cite journal |author=Sean Ferguson and [[Richard Parncutt]] |url=http://smc04.ircam.fr/scm04actes/P43.pdf |title=Composing in the Flesh: Perceptually-Informed Harmonic Syntax |access-date=2006-09-05 |archive-url=https://web.archive.org/web/20051013015208/http://smc04.ircam.fr/scm04actes/P43.pdf |archive-date=2005-10-13}}</ref> If the bass note also happens to be the chord's root, the interval's upper note almost always temporarily displaces the [[Third (chord)|third]] of any chord, and, in the terminology used in popular music, is then called a ''[[suspended chord|suspended fourth]]''.

Conventionally, adjacent strings of the [[double bass]] and of the [[bass guitar]] are a perfect fourth apart when [[open string (music)|unstopped]], as are all pairs but one of adjacent [[guitar]] strings under [[Standard tuning#Guitar family|standard guitar tuning]]. Sets of [[Tom-tom drum|tom-tom]] drums are also commonly tuned in perfect fourths. The 4:3 just perfect fourth arises in the C [[major scale]] between F and C.<ref>Paul, Oscar (1885). ''[https://archive.org/details/bub_gb_4WEJAQAAMAAJ <!-- quote=musical interval "pythagorean major third". --> A manual of harmony for use in music-schools and seminaries and for self-instruction]'', p.165. Theodore Baker, trans. G. Schirmer.</ref> {{audio|Just perfect fourth in scale.mid|Play}}

{{Listen|filename=Fourth_ET.ogg|title=Perfect fourth (equal temperament)|description=The file plays [[middle C]], followed by F (a tone 500 cents sharper than C), followed by both tones together.}}