Welcome to... "Micro-Lesson 261"
Mixolydian is the fifth mode of the Major Scale and is used in a good deal of Classic Rock songs by bands like: Lynyrd Skynyrd, Guns and Roses, and Neil Young.
The riff in this Micro Lesson plays through a popular Mixolydian harmony moving from the Tonic chord (D5), to the IV-chord (G5), and then into the VII-chord, (C5). The movement produces a great backdrop for this riff by highlighting the important notes of Mixolydian, (the lowered 7th and the major 3rd).
In measure one, the 5th string's, "D," root is performed as the tonal center to upper register inverted Power-chord ideas on 3rd and 4th strings. The major third of the key (F#), along with the lowered 7th (C), brings in measure two and moves up the modes harmony circling back to the initial riff in measure three.
Measure four uses some upper register tones to create a nice turnaround on strings 2 and 3, which once again highlight Mixolydian's major third and lowered 7th. Have fun learning this interesting and common sounding, (yet fairly easy to play), Mixolydian Rock riff in the, "D Mixolydian Mode." Enjoy!
Micro Lesson 261: "D Mixolydian" Classic Rock Riff