A horizontal hinge is a hinge that is mounted vertically on a door jam - but imparts a horizontal motion to the door. So, for us TGM'ers, the hinge is mounted on our lead (or left shoulder - for righthanded golfers) shoulder (again - vertically - just like a door jam) and this allows a horizontal motion of the clubface, controlled and guided by the flat left wrist. So.....hinging in TGM is the motion of the mandatory flat left wrist through impact - horizontal hinging is the left hand and hence, clubface, closes in relationship to the planeline. Like a door does.