The projective plane crossing number of a graph is the minimal number of crossings with which the graph can be drawn on the real projective plane. A graph with projective plane crossing number may be said to be a projective planar graph.

All graphs with graph crossing number 0 or 1 (i.e., planar and singlecross graphs) have projective plane crossing number 0.

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Weisstein, Eric W. "Projective Plane Crossing Number." From MathWorld--A Wolfram Web Resource. https://mathworld.wolfram.com/ProjectivePlaneCrossingNumber.html