Rings produced by cutting a strip that has been given half twists and been re-attached into equal strips (Ball and Coxeter 1987, pp. 127-128).
See also Möbius Strip
References
Ball, W. W. R. and Coxeter, H. S. M. Mathematical Recreations and Essays, 13th ed.
New York: Dover, pp. 127-128, 1987.