: What is this called? Why is it allowed? "Golden Train" by Justin Nozuka (YouTube link below) is in the key of Ab major. However he uses a Cb major chord throughout the song, which is not in the key of Ab major. It sounds

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The ?III is a borrowed chord from the parallel minor.

A bit more info: The bIII is commonly followed by the IV, giving it something of a subdominant function relative to the IV. The ii here is acting as a IV (it's the relative minor of IV) in a plagal cadence, so functionally what we have is more similar to I bIII IV, a common rock progression.

Also, the bIII can provide a particularly bluesy sound to an otherwise diatonic progression since it gives a minor pentatonic feel. Eric Clapton was fond of it as well.

Keep in mind that theory is descriptive, rather than prescriptive. Our brains are wired to hear certain sounds as consonant and dissonant, as creating or releasing tension, etc. There's a pretty good chance that Justin picked a bIII because it sounded good or because another song used it, rather than because he wanted to borrow a chord from the parallel minor. Nothing against Justin, he's a great songwriter. This is just how people usually work. However, knowing the theory does help broaden your palette of chord choices.