Pseudo-MOS scale

A pseudo-MOS scale⁠ ⁠^{[idiosyncratic term ]} is a ternary scale which is a MOS scale with a single step smaller than all the others, inserted at the end of the scale once per period. This "ultra-small step" is sometimes but not always equivalent to the aberrisma in aberrismic theory. The former large and small steps of the MOS are notated as L and m, and the new step is notated as s–pseudo-MOSes have a step pattern of the form xL ym 1s.

The name was coined by CompactStar. Pseudo-MOSes are simpler in structure than other ternary scales, because they are simply a compressed MOS scale with a pure octave at the end. However, importantly, the modes of a pseudo-MOS scale are not always pseudo-MOS (even if other pseudo-MOSes can be created from different modes of the original scale), and pseudo-MOS scales are not necessarily MV3. There is a distinction that is required between a pseudo-MOS's modes and the pseudo-MOS versions of the original MOS's modes, thus CompactStar has coined pseudo-modes for the latter. A pseudo-MOS scale exhibits Rothenberg propriety provided the MOS scale without the small step is proper.

Pinedye (5L 2m 1s) is the "pseudo-diatonic scale" and it is a simple example of pseudo-MOS scale. Not all 5L 2m 1s patterns are considered a pseudo-MOS, including the only one which is conditionally MV3, as it is not a diatonic mode with a small step at the end. In groundfault’s aberrismic theory, the small step of pinedye represents an 81/80 commatic inflection in the 5-limit untempered interpretation.