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Coplanarity requires both $I=I_d$ and $\Omega ={\varphi }_d$, up to the orbital-plane orientation degeneracy. Under that hypothesis the measured planet angle must obeyFor a nearly edge-on belt this offset is small except close to the projected crossings where $\cos f=0$. Measurements at several orbital phases can therefore test the entire projected ellipse: a systematic offset from the predicted relation implies a nonzero mutual inclination. A single projected location generally cannot establish coplanarity because $f$, the front-back branch, and the nodal orientation can be degenerate.

The oriented normal vectors to the planet and dust-belt planes areTheir inner product is the cosine of the mutual inclination, soIf ${\varphi }_d=\Omega$, this reduces to $\cos I_m=\cos (I-I_d)$; equal inclinations then give $I_m=0$. If both planes are face-on, it likewise gives $I_m=0$, independently of their undefined nodal longitudes. Reversing an observationally unidentified ascending node by $\pi$ produces the familiar orbital-plane orientation degeneracy for an axisymmetric belt.