Otherwise, for non-hyperelliptic "C" which means "g" is at least 3, the morphism is an isomorphism of "C" with its image, which has degree 2"g" − 2. Thus for "g" = 3 the canonical curves (non-hyperelliptic case) are quartic plane curves. All non-singular plane quartics arise in this way. There is explicit information for the case "g" = 4, when a canonical curve is an intersection of a quadric and a cubic surface; and for "g" = 5 when it is an intersection of three quadrics. There is a converse, which is a corollary to the Riemann–Roch theorem: a non-singular curve "C" of genus "g" embedded in projective space of dimension "g" − 1 as a linearly normal curve of degree 2"g" − 2 is a canonical curve, provided its linear span is the whole space. In fact the relationship between canonical curves "C" (in the non-hyperelliptic case of "g" at least 3), Riemann-Roch, and the theory of special divisors is rather close. Effective divisors "D" on "C" consisting of distinct points have a linear span in the canonical embedding with dimension directly related to that of the linear system in which they move; and with some more discussion this applies also to the case of points with multiplicities.