"Printed circuit boards are always composed of layer pairs. So there must be at least *two* layers in between. "

Wonderful.

I remember the days when I could look at the PC and actually draw a schematic of the circuit. Usually I could just "see" the circuit in my mind but some were a bit more complex so I had to draw it out.

It actually got easier when they started designing the more modern, rather than hand drawn. I could count the foils "from the big one" and add and subtract as needed.

It actually got easier than following the old point to point circuits which I used to do well. But now I am a bit lost. I find the old Tektronix type ceramic bindings a bit easier to follow, but not much. I guess I am just losing it.

I actually could follow a two sided board until everything got too small. I had good spatial reasoning and could turn the board, or me, to see the other side and lock my eyes to the point where I wanted to see.