Out of tune can be understood as a special form of tone sandhi, which belongs to the category of tone sandhi. Out-of-tune refers to the change of tonal color caused by the temporary change of tone or the temporary rise and fall sign in a musical work, and then returns to the original tonality after a short stay. This kind of harmony change is called out of tune. The sign of out-of-tune is often the existence of individual temporary tone sandhi in music works.

To put it simply, leaving the tune is to temporarily leave the original tune, neither consolidating nor developing on the new tune, nor ending on the I level of the new tune, but tone sandhi is consolidating, developing and ending on the new tune.