As I use this every day I go to work, I can concur with JimmyH...magnetic variation is the term....plus the old navigator's maxim:
Variation east, compass least, variation west compass best.
I also had a *very* old compass (now given to my son) that originally was my great great grandfathers (who was a Clipper ship Captain)....inscribed on the back: East or West, home is best
