We recently reviewed rock music's basic set of chords for a major key. There are six diatonic triads (chords built from the notes of the major scale), plus nine non-diatonic triads.
Yesterday we learned why the diatonic chord build on the seventh note of the scale, [seven]dim, doesn't get invited to the rock and roll party very often.
But there is one of those nine non-diatonic triads that's more widely used than any of the others, and it happens to be the "other seven chord," [seven]♭. In the key of C major, this is the B♭ chord. This chord is borrowed from the key of C minor, where it serves as the major alter ego for the minor [five]m chord.
You can use the [seven]♭ chord as a substitute for the dominant [five] chord. That is, you can take any of the basic three-chord phrases, and play [seven]♭ in place of [five]. The result is a new chord phrases that has the same harmonic functionality as the original, but with a different vibe. It's more earthy and tribal, less European and imperial.
The [seven]♭ is very popular in rock songs, especially in blues-based rock. Traditional blues melodies eschew the regular 7 note in major (the leading tone) in favor of the lowered 7♭ note, (the subtonic). If you don't sing the leading tone, then it's only natural to avoid chords, such as [five] that contain the leading tone. Of course, you can also go with [five]m, but throughout rock history [seven]♭ has been the favorite substitute dominant chord.
What do you get when you take a major scale and lower the 7 note to 7♭? I know that sounds like the setup for a clever joke, but the straight answer is, you get a different scale, the mixolydian scale. Music that uses the mixolydian scale is said to be in the mixolydian mode. (You can read the whole sordid and convoluted history of mixolydian mode at Wikipedia.)
Many rock melodies are written in mixolydian mode, or alternate between mixolydian and regular major (called ionian mode when you want to be precise about it). And that's just a fancy way of saying that rock melodies often use subtonic 7♭instead of leading tone 7.