Geometry of the problem for a prolate grain (here in cyan) spinning around B in perfect alignment. The axis is directed toward the observer. The magnetic field is inclined by an angle γ with respect to the plane of the sky . α is the spin angle of the grain around B, measured with respect to . The projection of the grain onto the plane of the sky is represented in gray. Point A is the intersection of the grain symmetry axis with the plane of the sky. The projected symmetry axis of the grain makes an angle ϕ with . The inclination of the grain with respect to the line of sight is ψ.