“Chords of inquiry” is Joni’s term for sus chords — which “suspend” (i.e., don’t play) the third of a triad and instead go down to play the second or go up to play the fourth. When you remove that third the chord itself also becomes as it were suspended between major and minor. It is ambivalent; it moves us to inquiry into its character.
Source: Court and Spark by Alan Jacobs