The rule underneath all three
Every central sp-carbon of a cumulene and every spiro carbon turns the next plane by 90°. Count these twisting centres:
ODD centres → ends ⟂no σ (a ≠ b) → OPTICAL
EVEN centres → ends ‖σ present → GEOMETRICAL
Quick count: even number of C=C or even number of rings → optical; odd → geometrical (cis/trans).
Biphenyls reach the same ⟂ shape through a rotation barrier (atropisomerism), not a twisting centre. So the test is: rings locked twisted and no σ.
