An (ordinary) torus is a surface having genus one, and therefore possessing a single "hole" (left figure). The single-holed "ring" torus is known in older literature as an "anchor ring." It can be constructed from a rectangle by gluing both pairs of opposite edges together with no twists (right figure; Gardner 1971, pp. 15-17; Gray 1997, pp. 323-324). The usual torus embedded in three-dimensional
![Torus -- from Wolfram MathWorld](https://cdn-ak-scissors.b.st-hatena.com/image/square/0e9d199124b73759f3b9521de077899dacaf484d/height=288;version=1;width=512/https%3A%2F%2Fmathworld.wolfram.com%2Fimages%2Fsocialmedia%2Fshare.png)