1. What is the difference between the diatonic and pentatonic scales?
This is a great question about music theory for guitar. The term diatonic basically means two tones. This refers to the two different kinds of steps that occur in major scales, whole-step and half-step. The word is also used to indicate when something stems from one key. For example, “this riff is all diatonic” means that all of its notes are straight out of the parent major scale. Or, “this guitar solo is not diatonic” means that it features notes that are not part of the parent major scale (like chromatic passing tones).
Pentatonic scale patterns are not considered diatonic because they are not based on a two step formula. And they don’t have seven notes like major scales do.
2. How can I play a major pentatonic scale over a song that uses chords from the major scale?
The pentatonic scale follows the root chord in a progression. The root is the tonal center of a song and usually where everything starts and ends. If the root chord is major, then you can play the same major pentatonic scale over the whole progression. If the root chord is minor, then you can play the same minor pentatonic scale over the whole progression.
For example, the song “Wonderful Tonight” by Eric Clapton is based on a 1 5 4 5 (G D C D) chord progression in the key of G. The G chord is the root since everything pulls to and resolves on it. So you can play G major pentatonic scale patterns over the whole song. Just position pentatonic pattern one so that the second note is G (the relative minor is E).
The guitar solo to “Stairway to Heaven” by Led Zeppelin is based on a 6 5 4 chord progression in C, with the 6 chord Am functioning as the root. So you can play Am pentatonic scale patterns over the whole progression. Just position pentatonic pattern one so that the first note is A.
In order to use the correct major scale, you must take the whole progression into account, not just the root. So in the first example play G major scale patterns. In the second play C major scale patterns (Am is the relative minor).