Neutral third scales
Neutral third scales are characterized by the division of the 3/2 "perfect fifth" into two equal "neutral thirds", which can be taken to represent either 11/9 or 27/22. This means the comma 243/242 is tempered out.
If the fifth harmonic is used at all, it makes sense to map it to 8 generators by tempering out 81/80, making it a meantone system, sometimes called "mohaha". If 7 is also mapped a particular way it is called mohajira. Alternatively, 64/63 could be tempered out, leading to a 126.96.36.199 version of maqamic temperament. But neither of these adjustments to the farther-out intervals affects its basic nature as a "neutral thirds" scale, which equally subdivides the 3/2 into two equal parts.
Any temperament tempering out 243/242 lends itself to neutral thirds; this becomes more significant when fifths are of low complexity, such as hemififths or the rank three temperament jove, but also includes miracle, harry and so forth. Nor does a scale need to be a MOS to qualify as a neutral thirds scale; that name could reasonably be given to Oktone, for example.
Basic 2.3.11 neutral thirds scale
In the 3L4s MOS, the large step is a 9/8 and the small step is a 12/11 neutral second. In the 7L3s MOS, the 12/11 is now the large step and the small step is a 33/32 quarter tone.
Though they differ in the specifics of their temperaments, all neutral third scales have certain munits in common.
- 4/3 Lss, 4/3 sLs: Rather than being divided into two large steps and one small step as in the diatonic scale, 4/3 can always be divided into one large step and two small steps. The large step is 9/8, and the two small steps are "neutral seconds" representing the sub-munit "32/27 ss".
- 11/8 LsL: In contrast, two large steps and a small step make a much less consonant and more colorful interval, 11/8.