How PCI-Express works (2020)
One neat thing about PCIe is that every PCIe compliant device must support using less lanes than it can. So your GPU which would like to work with x16 must support x1 etc.So in theory one could install a GPU or HBA in one of those x1 slots on the motherboard if one isn't terribly concerned about bandwidth. Except most of the motherboards use x1 connectors which doesn't have an "open back" for larger cards... so much for that[1].Another thing I didn't see it mentioning was this. Say you have a x16 slot on your motherboard, but you really want to install 4 NVME m.2 drives. Well each of those need a x4, but you have an x16 so per above you should be fine right, just plop in a "dumb" four socket m.2 PCIe card?Well, no, not unless the motherboard supports bifurcation[2]. If not, you'll need a card with a PCIe switch which can turn those four x4's into one x16.[1]: yes I know about risers[2]: https://www.10gtek.com/new-1414