There are seven different modes you can play on the guitar. 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.
Ionian (I)
Ionian is the first mode of the major scale — when the 1st scale degree functions as the tonic. Because it centers on a major chord (I), it’s considered a major key. It’s better known as the plain or relative major scale, and it’s one of the most commonly used modes.
Any type of chord progression that’s based in a major scale and centers on chord I is Ionian mode. Some chord progression and song examples include
“Twist and Shout” by The Beatles
D-G-A
I-IV-V in the D major scale.
D Ionian (better known as simply D major)
“Stir It Up” by Bob Marley
A-D-E
I-IV-V in the A major scale
A Ionian (better known as simply A major)
“The Lion Sleeps Tonight” by The Tokens
F-B♭ó-F-C
I-IV-I-V in the F major scale
F Ionian (better known as simply F major)
“Wonderful Tonight” by Eric Clapton
G-D-C-D
I-V-IV-V in the G major scale
G Ionian (better known as simply G major)
Aeolian (vi)
Aeolian is the sixth mode of the major scale — when the 6th scale degree functions as the tonic. Because it centers on a minor chord, it’s considered a minor key. In fact, it’s better known as the natural or relative minor scale.
Locrian (vii♭ó5)
Locrian is the seventh mode of the major scale — when the 7th scale degree functions as the tonic. As mentioned earlier, the 7th chord in the major scale has a minor-flat-five quality (diminished triad), which produces a dissonant and unresolved sound. Not many (any?) songs use it, so there’s no need to spend time on it here. Nevertheless, it’s still considered a type of minor key.