 Another curious feature you'll find recurring on PCBs is this little circle. It's called a fiducial, and it's a registration mark, a bit like those used to align ink layers in color printing. It's a target or reference point used by visual sensors on pick-and-place machines to properly place the board's components. A good PCB fiducial has no nearby circuit traces. It consists of a central copper disc surrounded by the exposed matte surface of the PCB substrate, and finally the board's solder mask. Multiple fiducials are better than one. You'll often find them located at opposite corners of a board in order to improve accuracy. It's like a little eye just lying there and staring back at you.