Multilayer PCBs greatly increase the area available for the wiring with one or more conductor pattern inside the board. This is achieved by gluing (laminating) several double-sided boards together with insulating layers in between. The number of layers is referred to as the number of separate conductor patterns. It is usually even and includes the two outer layers. Most main boards have between 4 and 8 layers, but PCBs with almost 100 layers can be made. Large super computers often contain boards with extremely many layers, but since it is becoming more efficient to replace such computers with clusters of ordinary PCs, PCBs with a very high layer count are less and less used. Since the layers in a PCB are laminated together it is often difficult to actually tell how many there are, but if you inspect the side of the board closely you might be able count them.
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