Originally Posted by
tomato coupe
Stack and reach implicitly reference the ground, as the ground is directly linked to the direction[s] of stack and reach measurements.
Exactly. I too was initially stuck on the truth that the bottom bracket axis and the top of the head tube axis are two fixed points with a constant straight-line distance between them which is not affected by the orientation of either flip chip. But the angle of that straight line does affect reach and stack.