When three or more notes sound at the same time, they form a chord. The most common type stacks notes in thirds, producing major and minor triads that serve as the basic building blocks of tonal harmony.

Chords create the sense of tension and release that drives much Western music. A dominant chord pulls toward the tonic; a minor chord can darken the mood. Guitarists and pianists often think in chords rather than single lines, using them to accompany melodies or to improvise.

Notation can show chords as stacked notes on a staff or as symbols above a melody line. Jazz and popular music rely heavily on chord symbols that leave the exact voicing to the player. Understanding chord construction and progression is fundamental for composers, arrangers, and improvisers alike.

Even simple songs gain depth when the chords move with purpose.