When a change of chord appears on a strong beat of the measure, it may extend into following beats. An exception occurs when a new phrase or motive begins on a strong beat, in which case the harmony of the previous weak beat may be repeated. Typically, there is no harmony at all. But, if the upbeat to a phrase is harmonized, the harmony may be repeated or changed on the strong beat that follows. Rhythmic syncopation is generally avoided in harmonic rhythm.
Syncopation results when stress is placed on weak beats rather than on strong beats.
This often occurs by accenting a weak beat or tying a weak beat into a strong beat. At times, however, an accented nonharmonic tone may appear. It may be better to let such tones remain nonharmonic, rather than trying to harmonize them, in relation to an overall better harmonic progression. When harmonizing an accented nonharmonic tone, the usual procedure is to omit the corresponding harmonic tone on the strong beat. The chord will be completed when the nonharmonic tone resolves to the chord tone.