I’m working on adding a mandolin line to “Nudie Cutie”. When I went to work on the chorus, I couldn’t remember what key it was in. I’d written down the tab notation, but not the actual notes or key. The two chords used in the chorus didn’t make any sense to me (although they sound really nice).
After an hour of poking around music theory sites, I finally identified them as Fadd9 and Gadd9, making the key (roughly) C. That makes sense since the verses are in G. In fact the transition from verse to chorus goes from G to Aadd9 to Gadd9, using the A to introduce the shift to 9ths.
Pretty clever for not knowing what the heck I was doing.