. . "In mathematics, the Klein bottle /\u02C8kla\u026An/ is an example of a non-orientable surface; it is a two-dimensional manifold against which a system for determining a normal vector cannot be consistently defined. Informally, it is a one-sided surface which, if traveled upon, could be followed back to the point of origin while flipping the traveler upside down. Other related non-orientable objects include the M\u00F6bius strip and the real projective plane."@en .