Comma pump
A comma pump is a JI or tempered chord progression whose starting and ending points differ by a comma.
There is some ambiguity in this term depending on whether the comma in question is tempered out (thus returning to the starting pitch) or not. If the comma is tempered out the chord progression could be called a comma loop.
Given a comma, a comma pump may be constructed by stacking root movements by basic intervals in the JI group in question to that comma. Reordering movements of a comma pump creates another valid comma pump progression for that comma.
Examples
Meantone
The I-vi-ii-V-I diatonic progression in Meantone diatonic is a Meantone or 81/80 comma loop:
CEG -> ACEA (C -> A: down by 6/5) -> DFA (A -> D: up by 4/3) -> GDGB (D -> G: down by 3/2) -> CEGC (G -> C: up by 4/3)
In JI and other non-Meantone tunings, attempting this comma pump results in the end point flatter by 81/80 relative to the starting point. In 21edo, since 81/80 is mapped to -1\21, this progression raises the pitch by 1\21.
Porcupine
Here's a Porcupine (250/243) comma loop, which only returns to the starting pitch in Porcupine tunings (^/v = 81/80 alteration):
C Ev G -> Av C Ev Av (C -> Av: down 6/5) -> Dv F Av (Av -> Dv: up 4/3) -> Bb^ Db^^ F^ (Dv -> Bvv = Bb^: down 6/5, Porcupine entails Dv = Db^^) -> G Bb^ D G (Bb^ -> G: down 6/5) -> C Ev G (G -> C: up 4/3)
