Is it really necessary to learn about guitar modes? Well, it’s not necessary to learn about anything. In fact, you don’t even need to learn how to play guitar. But we study things because they are interesting, provide enjoyment, and help us develop. Guitar modes is a confusion topic, Stillmode but once players figure out how they really work they are very glad they did.
Many guitarists have their own convoluted way of thinking about musical concepts. As a result their explanations about what they’re playing seem inconsistent with certain terminology. To say that Slash and Joe Pass don’t know anything about modal scales is inaccurate. Everything is in a mode. They couldn’t play music without them.
“Sweet Child O’ Mine” by Guns and Roses is a great example of Mixolydian mode. The two musical interludes that occur at 1:31 and 2:32 are based on the chords D, C and G which are V IV I (5 4 1) custom Built home in the G major scale (guitars tuned down 1/2 step). Although the parent major scale is G, it’s really the D chord that is functioning as the root.
Many guitar players make the mistake of basing the scale off of the root chord. But the D major scale won’t work quite right in this example because the C# note clashes with the C natural in the C chord. The correct way to play over this progression is to recognize that it’s a mode of the G major scale based on the fifth degree, D. This is called “D Mixolydian mode” (a.k.a. the Dominant scale).
So the correct major scale to play is G. If you want to apply the pentatonic however, you should follow the root chord, D. What you end up with is a combination of the G major scale and D major pentatonic.
Most guitar players favor the pentatonic boxes. So you could orient yourself in D major pentatonic first (pattern 1 starts at the 7th fret) and then mix in G major scale notes (G major scale pattern 3 overlaps D major pentatonic pattern 1). And this is exactly what Slash plays!