Editing Semitone (section)

==Augmented unison==
{{main|Augmented unison}}
[[File:Augmented unison on C.png|thumb|right|Augmented unison on C]]
[[File:Mendelssohn dominants.png|thumb|left|Augmented unisons often appear as a consequence of [[secondary dominant]]s, such as those in the soprano voice of this [[Sequence (music)|sequence]] from [[Felix Mendelssohn]]'s ''[[Songs without Words|Song Without Words]]'' Op. 102 No. 3, mm. 47–49.[[File:Mendelssohn dominants.mid]]]]
The '''augmented unison''', the interval produced by the [[augmentation (music)|augmentation]], or widening by one half step, of the perfect unison,<ref>Benward & Saker (2003). ''Music: In Theory and Practice, Vol. I'', p. 54. {{ISBN|978-0-07-294262-0}}. Specific example of an A1 not given but general example of perfect intervals described.</ref> does not occur between diatonic scale steps, but instead between a scale step and a [[chromatic]] alteration of the same step. It is also called a ''chromatic semitone''. The augmented unison is abbreviated '''A1''', or '''aug 1'''. Its inversion is the ''[[diminished octave]]'' (''d8'', or ''dim 8''). The augmented unison is also the inversion of the [[augmented octave]], because the interval of the diminished unison does not exist.<ref>Kostka and Payne (2003). ''Tonal Harmony'', p. 21. {{ISBN|0-07-285260-7}}. "There is no such thing as a diminished unison."</ref> This is because a unison is always made larger when one note of the interval is changed with an accidental.<ref>Day and Pilhofer (2007). ''Music Theory for Dummies'', p. 113. {{ISBN|0-7645-7838-3}}. "There is no such thing as a diminished unison, because no matter how you change the unisons with accidentals, you are adding half steps to the total interval."</ref><ref>{{Cite book
  | last = Surmani
  | first = Andrew
  | author-link = Andrew Surmani
  |author2=Karen Farnum Surmani |author3=Morton Manus
  | title = Alfred's Essentials of Music Theory: A Complete Self-Study Course for All Musicians
  | publisher = Alfred Music Publishing
  | year = 2009
  | pages = 135
  | id = Since lowering either note of a perfect unison would actually increase its size, the perfect unison cannot be diminished, only augmented.
  | isbn = 978-0-7390-3635-8 }}</ref>

[[Melody|Melodically]], an augmented unison very frequently occurs when proceeding to a chromatic chord, such as a [[secondary dominant]], a [[diminished seventh chord]], or an [[augmented sixth chord]]. Its use is also often the consequence of a melody proceeding in semitones, regardless of harmonic underpinning, e.g. '''D''', '''D{{music|sharp}}''', '''E''', '''F''', '''F{{music|sharp}}'''. (Restricting the notation to only minor seconds is impractical, as the same example would have a rapidly increasing number of accidentals, written enharmonically as '''D''', '''E{{music|flat}}''', '''F{{music|flat}}''', '''G{{music|doubleflat}}''', '''A{{music|tripleflat}}''').

[[File:Liszt augmented unison.png|thumb|right|[[Franz Liszt]]'s second [[Transcendental Études|Transcendental Étude]], measure 63]]
[[Harmony|Harmonically]], augmented unisons are quite rare in tonal repertoire. In the example to the right, [[Franz Liszt|Liszt]] had written an '''E{{music|flat}}''' against an '''E{{music|natural}}''' in the bass. Here '''E{{music|flat}}''' was preferred to a '''D{{music|sharp}}''' to make the tone's function clear as part of an '''F''' [[dominant seventh]] chord, and the augmented unison is the result of superimposing this harmony upon an '''E''' [[pedal point]].

In addition to this kind of usage, harmonic augmented unisons are frequently written in modern works involving [[tone clusters]], such as [[Iannis Xenakis]]' ''Evryali'' for piano solo.