View source for Chromatic semitone

{{Infobox interval|2187/2048|names=Chromatic semitone, augmented unison|name=Chromatic semitone, augmented unison|Name=Chromatic semitone, augmented unison}}

The 3-limit '''chromatic semitone''', also called the '''augmented unison''' ('''A1''') and represented by the ratio '''2187/2048,''' is the difference between the large and small steps of the MOS [[diatonic]] scale. It 

is generated by stacking 7 [[Perfect fifth|fifths]] and octave-reducing. In [[Pythagorean tuning|Pythagorean]] tuning (and thus purely-tuned [[just intonation]]), it is approximately {{Cents from ratio|2187/2048|ratio=2187/2048}} cents in size, but as an interval involved in the structure of the abstract diatonic scale it ranges from 0 to 240 cents depending on the tuning.

The chromatic semitone is significant in music notation. As it is the difference between, for example, the [[diatonic major third]] and [[diatonic minor third]], it is the interval represented by the # and b accidentals used in standard music notation. The chromatic semitone is distinct from the [[diatonic semitone]], which is the diatonic minor second and differs from the [[diatonic major second]] by a chromatic semitone.

As a harmonic interval, the chromatic semitone is a harsh dissonance due to its size, though tuning the diatonic interval qualities further apart (such as in [[arto and tendo]] theory) can alleviate that somewhat. In 5edo, it is a consonant 8/7 interval much like the [[diatonic major second]].

== Scale info ==
The chromatic semitone, being an interval with a fifthspan of more than 6, does not appear in the diatonic scale itself, but it is often an important interval to account for when discussing the tuning of diatonic.

== Just intonation ==
In just intonation, the chromatic semitone is a 3-limit interval. When tempered out, it leads to the whitewood temperament, where the MOS diatonic scale collapses to [[Equiheptatonic|7edo]].