<This was known by the trademark "Multiwire", and was often used for <quick-turn production. DEC used it on a few boards in the KL10-PV <processor; later revisions of the same boards were conventional PCBs. that was it. It's had two characteristics, high density and lines could cross (it was insulated). Since the "inner layers" were power and ground they were easy to modify (wires could be seen and removed or cut) and the transmission line characteristics were better than the then vogue 2 layer boards. All the lead holes were plated through as were any vias. Allison