[w3stie]

Originally Posted by Joe-Bob

Yes, it's a non-scale chord, but it's also not. It's what's called a Secondary Dominant. That means it's V7 of "X". In this case V7 of vi. Most, (but not all), chords in a key can have secondary dominants. It helps emphasize the arrival of that particular chord, even if it is only temporary.

Thanks Joe-Bob. I looked up Secondary Dominant on wikipedia. Sigh. It just gets more and more obscure