20. The _major_ mode is that of keys in which the tonic triad and the
two other cardinal triads are major.
21. The _minor_ mode is that of keys in which the tonic triad and one
other cardinal triad are minor.
22. A _diatonic scale_ is a series of the notes essential to one major
or minor key, arranged in order of pitch and repeating itself in other
octaves on reaching the limit of an octave.