# Steepest slope gradient of a vertical plane

I know the steepest slope gradient (Azimuth) of a 3D plane can be obtained by projecting normal vector onto XY Plane.

So, when the plane is slant, the steepest gradient will be a some value.

I am confused, when the plane is vertical but not parallel either to X or Y axis,

So, what would be the steepest gradient of my second case plane?

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The idea of a steepest slope gradient in the $X$-$Y$-plane presupposes that the plane is considered as the graph of a function $z=f(x,y)$. A vertical plane isn't the graph of such a function, so the concept of a steepest slope gradient doesn't apply to it.