The last of the fixed swing planes is the Squared Shoulder Plane. It’s the steepest of the three as its reference point is “...the point occupied by the right shoulder at the address.”(1)
I often see people pointing out a golfer’s swing plane angle is “too steep” or “too flat” when critiquing their stroke, but is that an accurate observation? Most of the time, no. As Mr Kelley points out: “No plane is technically too flat or too steep unless it allows the heel or toe of the club to touch the ground before separation...”(2) Again, this is why clubheads are built with rounded toes and heads, to stop that from happening and to accommodate different plane angles.