Major and minor
The root of a triad, together with the degree of the scale to which it corresponds, primarily determine its function. Secondarily, a triad's function is determined by its quality: major, minor, diminished or augmented. Major and minor triads are the most commonly used triad qualities in Western classical, popular and traditional music. In standard tonal music, only major and minor triads can be used as a tonic in a song or some other piece of music. That is, a song or other vocal or instrumental piece can be in the key of C major or A minor, but a song or some other piece cannot be in the key of B diminished or F augmented (although songs or other pieces might include these triads within the triad progression, typically in a temporary, passing role). Three of these four kinds of triads are found in the major (or diatonic) scale. In popular music and 18th-century classical music, major and minor triads are considered consonant and stable, and diminished and augmented triads are considered dissonant and unstable.