“Every single melody note works with every single bass note”
This is a line from a WIRED video featuring Jacob Collier and Herbie Hancock, but I’ve seen similar ideas espoused in various places. The concept is that for any given pair of bass and melody note, there exists a way to harmonize them. Put another way, there are 12 possible intervals that can exist between a bass and melody note, and each of those intervals fits with at least one chord.
This is a chart of the 12 intervals and all their possible harmonizations that composers can reference when looking to change up their harmony.