the secret chord

We have a gig in 3 weeks time; a fundraiser for Cancer Research UK.  It'll be my first since March last year, so Neil and I run through my songs before a full-blown rehearsal with the rest of the band.

He suggests a new arrangement of one of the songs and throws in a snazzy chord.  But forgets to tell me what it is.  And I forget to ask.  Hence the secret.