I understand that with the introduction and rise in popularity of PCI Express slots on motherboards, that more power was required to adequately run the adapter cards that used them (mostly high performance graphics cards);

therefore a new ATX power specification was introduced (ATX 12V version 2.2...?) that allowed for a 24-pin P1 connector (the main power connector to the motherboard) that was backward compatible with the older 20-pin connector.

I understand how the newer PSUs with the 24-pin connector could power a motherboard requiring only 20-pins (being backward compatible), but I have have seen pictures of 20-pin connectors coming from PSUs being plugged into a motherboard that had a 24-pin connector on it.

I'm guessing that it would be ok to use a PSU that only supplied a 20-pin connector on a motherboard with a 24-pin power slot available if you didn't plan to use any PCI Express cards, can anyone tell me if this is this correct? are there no steadfast rules when it comes to powering a system?

and just to confirm that the extra four pins, which apparently was added with ATX 12V version 2.2, are for an additional 12V of power, hence the name ATX 12V version 2.2... well what is ATX 12V version 2.1 or earlier

I'm finding the ATX power standards a bit confusing, any help with this would be much appreciated. Thanks!

Hey thanks for the reply, that sounds interesting. I guess actually experiencing these things first-hand is the best way to learn! Will give that wikipedia article a read, thanks.