A uniquely rock phenomenon is the common use of major triads or power chords (open fifths) built upon members of the pentatonic scale. In such a “tonal system,” songs make heavy use of I, flat-III, IV, V, and flat-VII. A related practice uses a different “rotation” of the pentatonic scale, replacing V with flat-VI.

For example, the chorus of The Beatles’ “Sgt. Pepper’s Lonely Hearts Club Band” is based on I, flat-III, and IV (all major triads).

Nirvana’s “Smells Like Teen Spirit” exhibits the “rotated” version of this system, repeating the power-chord cycle, I IV flat-III flat-VI.

Related to this minor-pentatonic system is the usage of Phrygian mode in heavy metal and hard core. In many of these songs, power chords are applied to the same minor-pentatonic scale degrees, with the addition of flat-II. An example is Shelter’s “The Message of the Bhagavat” (beginning at 0:14).