I was just messing around on my acoustic earlier and I realised how often I use a minor iv in a major key progression (so if C major was the key the chord I'd would use an Fm), and it got me thinking what other non-diatonic chords other might use in their progressions, so here I am asking
The time to hesitate is through..

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Quote by Draken
and the stig. American Stig would be a fat guy with a nascar hat.
I use that change a fair amount.
I-VIb (C-Ab)
I-III7 (C-E7)
I-v (C-Gmi)
i-VI7 (Cmi-Ab7)

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I like symmetrical movement of the same chord, like a 7sus4 moving in m3rds for example. Also works well with maj7 chords.