Chords are the building blocks of music, and they follow well-defined “rules”—but music would be very bland without notes that break free from those rules once in a while. In fact, melodic lines rarely remain strictly within the harmony. That’s where non-chord tones come in.
Non-chord tones, as you might guess from the name, are notes that are not part of the harmony at any given moment. They are used to either embellish or smooth out melodic lines. Because non-chord tones don’t fit in to the harmony, they also create dissonance, or harmonic tension, which is a crucial component of interesting music.
Balance is key: Non-chord tones are typically preceded and followed by notes that are part of the harmony (i.e., chord tones). Just like too much seasoning in a recipe might mask the flavors of the other ingredients, overuse of non-chord tones can confuse the ear’s sense of the overall harmony.
Here are some of the most common non-chord tones. Note: “Step” refers to either a half step or whole step, and “leap” refers to an interval of a third or greater.
A suspension is formed by holding a note into the next chord, before resolving down a step to a chord tone.
A retardation is the reverse of a suspension: One note is held into the next chord, and then resolves up a step to a chord tone.
An escape tone is formed by a step, and is resolved by a leap in the opposite direction. Think of it as a note that “breaks free” from the previous chord and then leaps into the new chord. Escape tones always occur off the beat.
A neighboring tone serves as an embellishment when the note in the melodic line stays the same from one chord to the next. It is formed by a step, and is resolved by a step in the opposite direction—that is, returning to the original note.
An anticipation can be formed by either step or leap, and is resolved by holding the note, which becomes part of the next chord. It’s as if the note can’t wait for the next chord and sounds early. Anticipations always occur off the beat.
An appoggiatura is formed by a leap (usually up), and is resolved by a step (usually down). Appoggiaturas always occur on the beat.
A passing tone fills in the empty space between two notes that are an interval of a third apart, creating smooth, scale-wise motion. It is formed by a step from the preceding note, and is resolved by a step in the same direction.
Really good info, images are broken though