[Yoda]

I understand that the "dual", or angled, hinge in dual horizontal or dual vertical hinging is meant to keep the clubshaft on plane while the clubface makes it's horizontal or vertical hinging motion. But, in 2-F, Homer states "All the action of the Golf Club takes place on a flat, inflexible, inclined Plane....Every other Component of the Stroke must be adjusted to comply with that requirement". So we already know that the clubshaft must be on plane. And in 2-G and 7-10, we learn that hinging is the motion of the clubface through the impact interval. Hinge action refers to clubface motion through impact and the incline plane deals with the motion of the clubshaft throughout the swing. If hinging is only dealing with clubface motion, why confuse the issue by adding the "dual" hinge and incorporating clubshaft (plane) motion? It seems to blur the identities of each.

Is there something I'm missing?

Hinge Action is Hinge Action and the Inclined Plane is the Inclined Plane. Their independent identities must be kept separate. You can execute any Hinge Action on any Inclined Plane Angle. Or, if you like, also on a Vertical Plane. And that is exactly why you must have available the optional Hinge Actions of Horizontal (only) and Vertical (only).

Explaining a bit further, when you 'cover' the Plane Line with your Clubface -- instead of merely 'point at' it -- you are executing a Vertical Plane of Clubhead Motion. This would normally be done only with Putting or short Chips. In this instance, you still must control the Clubface Plane of Motion by applying either Horizontal (only) or Vertcial (only) Hinging through Impact.

On the other hand, if you choose to 'point at' the Line, then you are operating on an Inclined Plane. This requires a primary hinge for Clubface control and a secondary hinge for the On Plane Clubshaft control. Hence, you are executing a Dual Hinge Motion, either Horizontal or Vertical. Since this is the most common application, it is common practive to use the terms Horizontal and Vertical Hinge Action interchangeably with their 'dual' counterparts.

Of course, there is no 'Dual' Angled Hinge Action because it is, by definition, executed on an Angled Plane of Motion.