The set of vectors in -space of norm 1 forms a sphere. Any particular vector , when rotated about the vector of means , defines a cone that intersects the sphere to form two spherical caps. The surface area of the caps divided by the surface area of the sphere equals the significance probability of the -test. Choose , the degrees of freedom; is then a vector in ()-space that has the -value corresponding to the other control. Although the sample vector lives in dimensions, a three-dimensional graphic is used to illustrate the cap-area phenomenon, with the cone angle determined by .