Bob, I'll try my best. I usually place the listeners in Boston or New York. .They are in Woburn and they get a call from their best friend to inform them that he won a million dollars, and will share it with them. But, they must get to Logan airport at exactly 6:00pm. So we set out for the airport and the first thing that happens is a traffic jam on RT93. We finally get through that jam only to get stuck in the Callahan tunnel. Well, to make a long story short, we finally get to Logan airport at 7:00pm, only to see our friend waving good-bye from the airplane as it taxi's down the runway. A week later, we get another call with the same type situation. But this time we're smarter, and contact the state police, to set up road blocks. These roadblocks get us throught the RT93 jams. We also have them clear a lane through the tunnel and control all the traffic lights to the airport. We get to the airport on time, and receive our windfall. What we really did was remove the impedances from our path, that prevented us from getting to point "b" at a particular time. Controlled impedance is very similar. There are a number of things, such as the properties of the materials selected (dielectric constant), the thickness of the copper, the distance between the traces and the reference plane, etc, which causes a signal to arrive at point "b", at a specific time. In todays high speed logic circuits, certain events happen a the presence of a signal, and other events happen at the absence of a signal. Designing for these types signals, requires a knowledge of materials and effects of the board manufacturing process. I hope this little tail helps. If you have any further questions, please feel free to contact me. Gary Ferrari Tech Circuits, Inc. Wallingford, Ct 203 269-3311