Consonance, also known as a concord, is a group of notes that are harmonious when sounded together as in a chord. Exactly which combinations of notes are considered consonant varies among different cultures and has changed considerably during the history of Western music. Definitions of consonance may also be found in acoustical theories from Pythagoras to Helmholtz. Intervals (the distance from one note to another) considered consonant in the common practice of tonal music are unisons, octaves, perfect fifths and fourths, and both major and minor sixths. Compare with dissonance (or discord).