 The only possible measurement is between any two orthogonal states (an orthonormal basis).So, for example, measuring in the rectilinear basis gives a result of horizontal or vertical.If the photon was created as horizontal or vertical (as a rectilinear eigenstate) then this measures the correct state, but if it was created as 45° or 135° (diagonal eigenstates) then the rectilinear measurement instead returns either horizontal or vertical at random.