The use of modal interchange in Judas Priest music (for musicians only!)
The chord of the fourth has these two uses in heavy metal music; either the higher or the lower tone is perceived as a chord root, which brings a very interesting harmonic ambiguity.
When the higher tone is perceived as the chord root, it usually works as an inverted fifth (the case of the known “power chord”). In that case, the harmonic series of the higher tone is an overriding element in perception. This is the case of the guitar riff in Deep Purple’s “Smoke on the Water”
Another example of the use of the perfect fourth in the song “Devil’s Child” of Judas Priest (part of the album “Screaming for Vengeance”, 1982) has an E-Dorian I5 -IV4–3-I.
If you are interested about modal interchange or modal mixture, I have an article that expands this topic:
Another mode that was used by Judas Priest was the Aeolian mode that was taken by heavy metal through the American folk revival of the 1960s. In this example we have the progression used by the singer and composer, Joan Baez:
an Aeolian I-VI-IV-VII-I progression that is very common in this mode. The song is called “Diamonds and Rust” (Diamonds & Rust 1975) and was arranged by the band.
You can have another examples of how to use these kind of modes here (for musicians only, as it is fairly technical):