(On a PDP, the backplane is the panel into which all circuit boards are plugged). In the PDP-8, the backplane is mounted vertically and the connector slots for the circuit boards are also vertical. Most circuit boards have 18 connectors and plug into one connector slot. Some boards are siamesed, have 36 connectors and occupy two slots, one below the other. These are known as "double height".
DEC changed the orientation of the backplane and the connector slots in later models but always referred to the boards as if they were mounted in this way, Modules are referred to as double height, quad height, hex height, whether they plug in horizontally, vertically or straight downwards. This causes confusion.
The modules, (circuit boards), fall unto several classes. The original PDP-8 uses largely standard DEC logic modules, the so-called R series (these are fitted with RED handles). The PDP-8 also uses other standard modules, W series (WHITE handles), B series (BLUE handles) and G series (GREEN handles). There are also A series (AMBER handles); these are only found on machines that have an a/d converter fitted.
Some of the R series boards were not fast enough for the PDP-8 and were modified to increase their speed. These became the S series (these also had RED - ?perhaps SCARLET? handles).
There is sometimes confusion about R & S series modules. These are identical excepting that some resistor values are changed in the S series to make them operate faster. The PCBs and all other components are the same and module for module they perform the same logical function as the R series.
These modules are all single sided, approximately 2½ by 5 inches, with 18 connection fingers on the card. There are, of course, the inevitable exceptions. R210, the accumulator, is double height and these cards occupy two back plane slots, as do R211, G209 and W300. These double height modules are approximately 5¼ by 5 inches.
These modules are the same physical size as the earlier R series but the connection fingers are plated on both sides of the PCB making 36 connectors per board. The boards are populated with small scale integrated circuits. M series boards have MAGENTA handles.
This M series is not the same as the last one. The PDP-8/E was a radically different design and introduced the "omnibus" backplane. This allows for boards which are four times the width (4 sets of connectors) and are also rather longer (10½ by 9 inches). These are referred to as quad height, even though the backplane is now mounted horizontally and the modules plug into the backplane from above. These modules have MAGENTA handles.
PDP-8/A models use M series boards. These are different again, because the PDP-8/A has a different backplane, this time potentially six slots wide (high!). The modules are, of course, referred to as being hex high. The backplane is mounted vertically (as per the original 8), but the boards plug in horizontally. The M series modules used in the PDP-8/A have, guess what, METAL handles! To be fair, this is necessary to support that length of PCB when it is mounted horizontally (they still sag though).
Although the PDP-8/A backplane can potentially accommodate six slots, in practice, this does not happen. Some 8/A backplanes (in machines with semiconductor memory) have only four slots and some are a mixture of four and five slots. Core memory boards use five slots.