Torus

u,v are in the interval [0, 2π),

R (or A) is the distance from the center of the tube to the center of the torus,

r (or a) is the radius of the tube.

R and r are also known as the "major radius" and "minor radius", respectively. The ratio of the two is known as the "aspect ratio". A doughnut has an aspect ratio of 2 to 3.