Non-Chord Tones
The notes that don't belong to the underlying chord — where melodies get their color and forward motion.
If every melody note had to belong to the chord underneath it, music would sound like a series of arpeggios. Non-chord tones — notes borrowed briefly from outside the current harmony — are what let a melody move by step, add tension, and feel like a line rather than a broken chord.
A passing tone fills the gap between two chord tones a third apart, moving through stepwise (C-D-E over a C chord — D is passing). A neighbor tone steps away from a chord tone and immediately back (C-D-C — D is a neighbor). Both are typically unaccented and resolve by step, so they read as decoration rather than disruption.
A suspension is stronger: a note is held over from the previous chord into a new one, landing on a beat as a real dissonance, and then resolves down by step once the harmony has already changed underneath it — this delayed resolution is one of the most direct tension-and-release devices in tonal music. An appoggiatura is similar but arrives by leap instead of being held over, landing hard on the beat before resolving by step.
A pedal point works at the opposite scale: instead of a single note against a chord, an entire sustained bass note holds still while the harmony above it changes freely, including through chords that would normally clash with it — a favorite device for building sustained tension toward a climax or cadence.