The phase plane essentially is a plot of heat conduction versus temperature. Given the phase plane of a particular class of extended surfaces, the engineer can quickly execute numerous tactics of design analysis or synthesis, because the plot displays heat dissipation, temperature distribution, and reduced length all on one diagram. A special set of coordinates is proposed, and their universality to all extended surfaces is explored. Working diagrams are presented for straight rectangular and triangular fins. Some numerical examples show how to use the diagrams.