Pythagorean comma - interval between 2/1 and (3/2)^12 up to octave equivalence.
If you play Db Ab Eb Bb F C G D A E B F# C# as pure 3/2 fifths, then the C# is slightly sharper than the Db, by a Pythagorean comma
Syntonic comma 81/80 - the interval between 5/4 and (3/2)^4 up to octave equivalence.
If you play C G D A E using pure ratio fifths for the intervals, then drop back to C using a pure ratio major third, 'the C you reach is a little sharper than the starting pitch, by a syntonic comma.
Or other very small interval between low number ratios or products of ratios.
for Robert Inventor's Programs