A major problem with the performance of music in just-intonation, especially on unaccompanied non-fixed-pitched instruments (i.e., string quartet or a capella vocals), is the possibility of the overall tonality drifting in commatic increments, usually downward ('flat').
The problem occurs mainly in diatonic music, and because of the ambiguity of the 2nd degree of the major scale. This degree normally needs to be tuned to either of two ratios in order to fit harmonically all of the chords in the scale: as a 9:8 to fit the V ('dominant') chord and its relatives, and as a 10:9 to fit the ii ('supertonic') and IV ('subdominant') chords and their relatives.
See Monzo, An Examination of Fox-Strangways and Partch on JI modulation (under the section "Further commentary by Monzo") for examples (with audio files) of commatic drift.
a shift is an immediate change in the pitch of a note, as the note is held or repeated from one harmony into another.
a drift is an overall pitch change of the entire scale. its effect on the pitch of any note doesn't become evident until an entire "comma pump" chord progression has been traversed.
for example, in the classic problem of rendering the I-vi-ii-V-I progression in strict JI, one either has a shift (the 2nd scale degree shifts from 10/9 in the ii chord to 9/8 in the V chord) and no drift, or a drift (the final I is lower by 80:81 than the initial I) and no shifts.
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