Because of its positive Lyapunov exponents, an isolated chaotic system synchronizes with nothing else. The synchronization of two or more chaotic systems is actually another version of the usual thought experiment where one imagines two initial conditions starting close to each other in phase space. For two separated chaotic systems, trajectories starting at close initial condition will diverge at first. It was counterintuitive to have the two initial conditions case converge to the same trajectory. We now realize that two identical uncoupled chaotic systems have twice as many positive Lyapunov exponents as either of the systems by themselves. If, on the other hand, we add some sort of coupling between the isolated systems, we can alter the Lyapunov spectrum-depending on the coupling and the specific systems, we could decrease or increase the number of positive Lyapunov exponents. In our original synchronizing system, we went from a total of two positive Lyapunov exponents in the combination of two uncoupled systems to one positive Lyapunov exponent in the pair of coupled systems. We show below how we originally handled this situation mathematically so we could calculate the stability of the synchronized state. As we go through this article, we will also show how this approach matured and became general with a geometric way to view the synchronization in large networks of coupled oscillators.