A simple diagram illustrates the relations between six parameters of the generalized FRT. The diagram is developed to illustrate a variety of equations which have appeared in the literature, particularly in relation to free-space propagation, Gaussian beams, optical filtering experiments, and the ABCD matrix. Analysis of the effect of imperfect collimation and of the production of a Radon-Wigner spectrum using a varifocal lens are given as examples of the technique.