Jerry . . . Vertical in this context means 90 degrees . . .
So horizontal hinging would be holding your left wrist vertical to the ground while you move your arm back and forth like a door to your house or a helicopter
vertical hinging would be holding your wrist vertical (90 degrees) to a vertical plane . . . your arm would move like an attic door . . .
angled hinging would be holding your wrist vertical to the inclined plane . . . like a paddle or one of them bomb shelter doors.
So the hinge pin . . .which is in your shoulder is mounted VERTICAL . . 90 degrees . . . to the plane of motion . . . but with vertical and horizontal hinges you have to have ANOTHER HINGE to lay the motion ON THE INCLINED PLANE . . that's why you have DUAL HORIZONTAL AND DUAL VERTICAL . . . dual representing two hinges . . . one hinge perpendicular to the associated plane (horizontal plane or vertical plane) and the other hinge to lay the blade of the hinge (left arm) on the INCLINED PLANE . . . See the pictures 10-10-C thru E . . . you'll see what I'm talking about . . . you DON'T need the second hinge to lay it on the plane with angled so you don't have "dual" angle . . .no need for the second hinge because the motion is already on the inclined plane.
Hinging is actually with the WHOLE ARM . . that is the full blade of the hinge . . .Homer just focused on the wrist because it could be verified in terms of what it was vertical (90 degrees) to . . . (ground, wall, inclined plane) . . . you'll note in the 10-10-C thru E pics the two theoretical hinges in the left shoulder . . . one is like a door hinge the other is like a lose nut and bolt that allows the blade of the hinge (left arm) to be laid on the plane.
but to answer your question . . . vertical is vertical to the ground with horizontal . . but 90 degrees to a wall with vertical hinging and 90 degrees to the golf plane (roof) with angled hinging . . .
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