Major and Minor Keys
Musicians often categorize songs as either being major or minor, but the truth is that there are three different major mode keys and four different minor mode keys. You need to figure out if a “major” song is Ionian, Lydian or Mixolydian mode. You need to figure out if a “minor” song is Dorian, Phrygian or Aeolian mode (the other, Locrian, isn’t used).
Relative Major and Relative Minor
The two most commonly used modes are Ionian and Aeolian, and they are the ones also called the relative major and relative minor. Ionian, the relative major, stems off of the first scale degree in the major scale and Aeolian, the relative minor, stems off of the sixth. But there are other scale degrees that you can build a chord progression off of.
Figure Out the Mode
When playing over a song, you figure out the mode by analyzing the whole chord progression and determining which parent major scale the chords all fit into together. Once you determine the proper parent major scale, play those major scale patterns over the whole progression. If a progression fits into a key and revolves around the first degree, then it’s Ionian mode, which is also the plain major or relative major. If a progression fits into a key and revolves around the sixth degree, then it’s Aeolian mode, which is also the natural minor scale or relative minor. But there are more possibilities. If a progression revolves around the second it’s Dorian, third it’s Phrygian, forth it’s Lydian and fifth it’s Mixolydian (the seventh, Locrian, isn’t used). As long as you play the same major scale that the whole chord progression fits into, then you’re playing the right notes. But the root will be different depending on which scale degree everything is revolving around. So pay attention to the mode root and emphasize it (you can also emphasize the notes in the root chord of the progression).
Applying the Pentatonic Scale
While we’re on this topic, I should point out that you do not need to figure out the parent major scale when applying the pentatonic scale patterns. The pentatonic can simply follow the root chord regardless of which mode it actually is. For example, if you have a progression that revolves around a G major chord, then you can play G major pentatonic over the whole progression regardless of what the other chords are. If you have a progression that revolves around an E minor chord, then you can play E minor pentatonic over the whole progression regardless of what the other chords are.
Some other helpful blog posts on major and minor tonalities and guitar modes are: