Projective planar graphs can be characterized by a set A of 35 excluded minors. However, these 35 are not equally important. A set E of 3-connected members of A is excludable if there are only finitely many 3-connected nonprojective planar graphs that do not contain any graph in E as a minor. In this article, we show that there are precisely two minimal excludable sets, which have sizes 19 and 20, respectively

Archdeacon

Ding

Glover

Hall