English: Central part of nonorientable Boy surface, flattened with folds along sides of a regular triangle, cut out of paper. Note the triple intersection in the center, double intersection lines from the center to midpoints of the sides of the triangle and fold annihilation at the vertices. Outer edge can be extended to infinity (flat version) or glued to a sphere (standard Boy surface)

English: Central part of nonorientable Boy surface, flattened with folds along sides of a regular triangle, cut out of paper. Note the triple intersection in the center, double intersection lines from the center to midpoints of the sides of the triangle and fold annihilation at the vertices. Outer edge can be extended to infinity (flat version) or glued to a sphere (standard Boy surface)