There’s a broad diversity of harmonic practices being used out there in the world of blues-based popular music, rock in particular. While a given song may not use a lot of scales and chords, the relationships between those scales and chords is rarely simple or obvious. You really just need to learn all of them. It takes a lot of practice. Fortunately, there is a single scale that works in every situation, which I’ll get to at the end of this post.
First we need to understand what a key even is. In Western classical music, the idea is pretty straightforward. There are two kinds of keys, major and minor. Each major key is generated entirely from a major scale. The C major scale is C, D, E, F, G, A, and B. If you pick any note from the scale and then go up skipping every other note, you get a chord. The seven chords you can get from the C major scale this way are C major, D minor, E minor, F major, G7, A minor and B diminished. Taken together, these are the chords that comprise the key of C major. Because they consist entirely of notes from the C major scale, you can use the C major scale over all of them, and everything will fit. Be aware that not every note will sound equally good in every situation, you need to use your ear for that.
Minor keys are more complicated. The C natural minor scale is C, D, E-flat, F, G, A-flat, and B-flat. If you generate chords from C natural minor, you get C minor, D minor, Eb major, F minor, G minor, Ab major, and Bb7. You can play C natural minor over any of them and be in good shape. However, this collection of chords is unsatisfying for various reasons. The big problem is that they sound more like E-flat major than C minor. You really want that G minor chord to be a G7. To produce it, you need another scale, the C harmonic minor scale: C, D, E-flat, F, G, A-flat, and B. That last note is the important one. The chord you get if you start on the G in the C harmonic minor scale is the one that really makes C minor feel like C minor. So when you see G7 in the key of C minor, raise your B-flat to B.
You will quickly discover that real-life music rarely sticks exclusively to the major and minor keys. There’s a common device called modal interchange, where you have a major-key song that borrows from the parallel minor key, or vice versa. In C major, your song will sound a lot more interesting if you bring in chords from C minor. It’s also pretty common to have a song that’s in C minor everywhere except for the actual root chord, which will be C major instead. It sounds like a weird idea, but it’s a very common songwriting technique. If you really memorize your major and minor keys well, then you can learn to recognize when a chord comes from C major and when it comes from C minor, and change scales accordingly.
That takes care of standard practice from classical music. But now you need to deal with a whole other harmonic universe, the modes. A very common mode for rock and pop is Mixolydian. The C Mixolydian mode is C, D, E, F, G, A, and B-flat. The chords you get from this scale are C7, D minor, E diminished, F major, G minor, A minor, and Bb major. So if you see a song that starts and ends on C7, and that has Bb chords in it, you need to use your C Mixolydian to sound correct. In rock, Mixolydian is more common than plain major, so you should expect to see it a lot.
Over on the minor side, the most common mode you see is Dorian. The C Dorian mode is C, D, E-flat, F, G, A, and B-flat. Now your chords are C minor, D minor, Eb major, F7, G minor, A diminished, and Bb major. If you see an F7 chord in C minor, that means you need to use your Dorian scale.
There are many more scales and modes. Phrygian mode is a minor scale that has a Flamenco or Middle Eastern feel. Lydian mode is a major scale with a floating, dreamlike quality. Jazz and the brainier forms of metal use the melodic minor scalewhose chords and modes are wonderfully freaky. Fortunately, none of these scales are as common in rock and pop as the ones above.
But wait! What if the key changes? What if the song that’s been chugging along in C during the verses and choruses suddenly lifts up to Eb during the bridge? What if it goes from C Mixolydian in the verses to G Mixolydian in the choruses? What if it has more of a jazz feel and changes keys once a measure? The best you can do in these situations is to try to group the chords together into keys or modes as best you can, and be ready to use the appropriate scale for each passage.
You might be feeling some despair at this point. But there’s some good news. You have three useful shortcuts: the major pentatonic, the minor pentatonic, and the blues scale.
C major pentatonic is C major without the fourth and seventh notes: C, D, E, G, and A. You can use C major pentatonic in any major or Mixolydian situation and it will probably sound fine. It doesn’t have any dissonant notes in it, so you can’t really play anything “wrong.” You also can’t play anything super interesting, but at least it’s a good way to get your feet under you.
C minor pentatonic is C natural minor without the second and sixth notes: C, E-flat, F, G, and B-flat. You can use it in any natural minor, harmonic minor or Dorian situation, and once again, you won’t be able to do anything that sounds terrible. Like its major cousin, minor pentatonic gets a little bland if you overuse it, but it’s perfect for beginners. Both minor pentatonic scales are easy to play on the guitar, using a fingering known as the pentatonic box.
Finally, there’s the lead guitarist’s best friend, the blues scale: C, D, E-flat, F, F-sharp, G, and B-flat. The blues scale sounds good over the blues, obviously. It forms the basis of blues tonality, which is a category distinct from major or minor, sharing aspects of both. But here’s the really cool thing: the blues scale sounds good in any harmonic situation. If your song is in C major or minor or any of the modes, you can just blow over the whole thing with C blues, regardless of what happens underneath, and the results will sound fine. This method is easy to run into the ground, but there are a great many lead guitarists who have made long and fruitful musical careers doing nothing else.
