Chords constructed of three notes of some underlying scale are described as "triads". Chords of four notes are known as "tetrads", those containing five are called "pentads" and those using six are "hexads". Sometimes the terms "trichord", "tetrachord", "pentachord", and "hexachord" are used—though these more usually refer to the pitch classes of any scale, not generally played simultaneously. Chords that may contain more than three notes include pedal point chords, dominant seventh chords, extended chords, added tone chords, clusters, and polychords.