Rhythmic emphasis placed on normally unaccented beats or subdivisions, creating tension against the predictable grid.
Syncopation is the placement of rhythmic emphasis on beats or subdivisions that are normally unaccented in a measure, such as the "and" of a beat or the offbeat sixteenth notes between main pulses. By landing accents where the listener does not expect them, syncopation creates forward tension and a sense of momentum that pulls against the underlying grid.
Why it matters
Understanding where a track's syncopation falls tells a DJ whether two tracks will lock together rhythmically or clash when layered. Heavy syncopation in a bassline or percussion part can mask a clean transition if the incoming track sits squarely on the grid.
In practice
When blending a highly syncopated track with a straight four-on-the-floor track, let the EQ-heavy elements of one track drop out before the other's syncopated parts become audible. Overlapping both simultaneously often creates rhythmic mud.

