Let K be a maximal arc with degree d. Consider the incidence structure , where P contains all points of the projective plane not on K, B contains all line of the projective plane intersecting K in d points, and the incidence I is the natural inclusion.
Consider a maximal arc K within the projective plane, with degree d.  The incidence structure involves two sets: P (all points outside K) and B (all lines intersecting K at d points). The incidence relation I maps each point in P to every line in B.