In Chapter 9 we talked about how current flowing along traces can radiate EMI. In Chapter 10 we talked about how traces couple to adjacent planes, how that relates to return currents, and then to impedance control. In Chapter 16 we will talk about differential traces coupling to each other and the implications that has for signal integrity and impedance control. All of those coupling mechanisms are essentially the same; they rely on the facts that (a) like charges (electrons) repel each other, and (b) flowing electrons can create magnetic fields that induce currents in other conductors. In this chapter we look more closely at the specific coupling mechanism that can occur between adjacent traces on the board itself.