I'm on the road, and away from my documentation, but I definitely would not call MUD 'niche' - it's pretty much the standard hex-height UNIBUS spec in all the later UNIBUS backplanes. Here's what I can recall: in the beginning, there were what were called 'Small Peripheral Controller' (SPC) slots, which took quad-high cards in the C-F connectors. Older, simpler interfaces (think DL11, RX01/2 controllers, etc) were quad cards which went in the C-F connectors of an SPC slot. (More complicated interfaces, like the RK11, RP11, DH11 etc were entire custom backplanes/system units - 4 slot for the RK11, larger for the DH11, IIRC.) I forget what was in the top two connectors (A-B) of the old SPC slots (e.g. on the early DD11s - the name of the early 4-slot backplane units). Of course, in the 1 and 4 slots, the A and B connectors had 'standard UNIBUS' so one could plug in either a jumper module to the next backplane (earlier M920, later M9202 - once they started dealing with lumped loads), or a UNIBUS cable (the big flat white one) to another system unit. So then hex cards started to become common (and basically all later UNIBUS interface cards are hex cards - from early ones like the DZ11 and RL11, to the latest ones like the UDA50, DELUA, etc). Around then, the MUD spec came in: in a MUD slot, the bottom 4 connectors (C-F) are still SPC (so you can plug any quad SPC board into the C-D connectors of a MUD slot), but the top two connectors (A-B) are subtly different. I _think_ many (most?) hex boards are MUD only, but I would have to check on that. When I get back, I will check and see what the A-B connectors in an old SPC slot are - and if any hex UNIBUS cards use that, instead of MUD. (Of course, any hex card that _only_ uses SPC pins can go in either kind. I don't know if there are any hex cards are like that - or perhaps many are, I just don't know off the top of my head.) Well, I can kinda-sorta understand the UNIBUS and QBUS not being safe from each other. It's sort of 'intuitively obvious' that one shouldn't plug a card dual-high cards, except the LSI-11 CPU card, another differentiation.) But the Q/Q and Q/CD thing is, to me, at least, different - one has to know there are two kinds of QBUS, and not to plug cards for one into the other (except the many cards that can go both ways). But now that I think about it, I guess there are sorta two kinds of UNIBUS too - MUD and non-MUD. And I think I even recall someone damaging a card by plugging a non-MUD card into a MUD slot (or vice versa, I don't recall the details any more). Noel