0. Introduction. A surface of order three, F, in the real projective threespace P 3 is met by every line, not in F, in at most three points. F is biplanar if it contains exactly one non-differentiable point v and the set of tangents of F at v is the union of two distinct planes, say τ 1 and τ 2. In the present paper, we classify and describe those biplanar F which contain the line τ 1 ∩ τ 2.

We describe a surface by determining the tangent plane sections of the surface at the differentiable points. This approach was introduced in [1] and it is based upon A. Marchaud's definition of “surfaces of order three” in [4].