Tetrachord, in music, refers to a series of four notes that are played in a sequence. Synonyms for the word tetrachord may include quartal harmony, four-note chord, and four-note scale. Quartal harmony refers to a musical structure where chords are constructed using fourths instead of thirds. Four-note chord is a term that describes a harmony made up of four different notes and is often used in jazz music. Four-note scale describes a sequence of four different pitches that make up a musical scale. Overall, these synonyms for tetrachord are useful when discussing different musical structures and scales that incorporate four notes.