A chord is a set of three or more notes. Chords are usually made up of intervals, most commonly major thirds and minor thirds. Chords are also derived from scales and are used to provide harmonic accompaniment to the melody of a song. Chords are essential to providing harmony for a song. For example, when a pianist or guitarist accompanies a singer, he uses chords to create the harmonic accompaniment. By learning how chords are constructed and how different chords work together, you will start to recognize and remember the common chord progressions. You will also have a strong foundation for composition and improvisation.