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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.