Recently I wrote about wiring wall outlets. This time I want to talk about wiring wall switches and light fixtures. If you've ever looked at an installed wall switch, you might see that there are two black wires attached to it, and no white. Or you might see a black and a white. Or there might be a black and a white with a black mark on it. Does it make a difference? And if so, what is the difference? Let's find out.