Relative major and relative minor aren’t the only types of keys you can have when playing the guitar. In fact, any degree in the major scale can function as the tonic (or key) and serve as the starting place in the scale, so because the major scale has seven degrees, it also has seven possible starting points, or modes.
In a major scale, three degrees produce major triads and chords — I, IV, and V — so any one of these can be the mode of a piece of music. The major scale also has three degrees that produce minor chords — ii, iii, and vi — and each one of these can also be the mode of a piece of music.
Notice that the 7th scale degree is not mentioned as a modal option. The 7th chord in the major scale has a minor-flat-five quality (diminished triad), which has a dissonant and unresolved sound. The instability of its sound makes it impractical to base a piece of music on it.
Each mode of the major scale is identified by a Greek name:
Ionian (I)
Dorian (ii)
Phrygian (iii)
Lydian (IV)
Mixolydian (V)
Aeolian (vi)
Locrian (viif5)
