FTP name this attempt to unify Euclidean and non- Euclidean geometries, which resembles a trumpet with the mouth-piece doubled back.
Effectively, it is a proof that every individual geometry can be constructed projectively. Constructed by the man who in 1895 initiated the writing of an encyclopedia of mathematics, it is a one-sided surface which is closed and has no boundary. FTP name this attempt to unify Euclidean and non- Euclidean geometries, which resembles a trumpet with the mouth-piece doubled back.
Effectively, it is a proof that every individual geometry can be constructed projectively.
{Klein} bottle