duocylinder

right|frame|Stereographic projection of the duocylinder's ridge (see below), as a [[flat torus. The ridge is rotating about the -plane.]]

The duocylinder, also called the double cylinder or the bidisc, is a geometric object embedded in 4-dimensional Euclidean space, defined as the Cartesian product of two disks of respective radii r1 and r2:D = \left\{ (x,y,z,w) \, \left| \, x^2+y^2\leq r_1^2,\ z^2+w^2\leq r_2^2 \right. \right\}