A chord is any combination of three or more pitch classes that sound simultaneously.
A three-note chord whose pitch classes can be arranged as thirds is called a triad.
To quickly determine whether a three-note chord is a triad, arrange the three notes on the “circle of thirds” below. The pitch classes of a triad will always sit next to each other.
A triad (A, C, E) on the diatonic circle of thirds.
