IMO Hardy's so called 'one plane vs two plane' book is 'actually' not about plane at all, but a quite solid description of the differences between spine angle/tilt at address and the resulting swings/compensations they each require.
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"Support the On Plane Swinging Force in Balance"
"we have no friends, we have no enemies, we have only teachers"
Simplicity buffs, see 5-0, 1-L, 2-0 A and B 10-2-B, 4-D, 6B-1D, 6-B-3-0-1, 6-C-1, 6-E-2