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Why are higher-lane PCIe cards mechanically incompatible with lower-lane slots? For example, why are the x8 slots designed in such a way that an x16 card can not be inserted into them? They could have just left the back of the slot open so that any size card could be inserted. Aren't such devices compatible electrically?

Edit: To those arguing that cards have certain bandwidth expectations, and the mechanical restrictions enforce them: that's not true.

Just because a slot is x16 width, doesn't mean that an x16 card will be able to use 16 lanes: The slot could be x8 electrically, or the number of available lanes can be automatically limited from x16 to x4 because of other PCIe cards inserted into other slots.

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  • Because the devices wouldn't work as designed without the additional lanes.
    – Ramhound
    Commented Jul 26, 2014 at 23:50

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It wouldn't be safe to insert e.g. an x16 card into an x8 slot, because they have different power requirements and can cause an electrical overload: an x16 card can draw up to 75w from the board, while an x8 card is limited to 25w.

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  • I would say this is not strictly true. I have modified x1 slots to accept (very low end) graphics cards before, and everything works just fine.
    – alex.forencich
    Commented Oct 30, 2019 at 6:11
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    @alex.forencich there are people who survive car crashes while not wearing seat belts ....
    – MWB
    Commented Oct 30, 2019 at 16:45
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Electrically compatible - tough to say.

A PCIe x1 has 1 high speed data channel. A PCIe x16 has 16 high speed data channels (therefore it needs more connectors on the card and slot). When a PCIe x16 card is forced into a PCIe x1 slot, the card can only use 1 data channel, even though it is requesting to use more. I would not be surprised if some video cards required more than 1 data channel to work properly.

One of the reasons that cards with more data channels don't fit into the slot is that the specifications for PCIe x1 were written before PCIe x2, which were written before PCIe x4, ...x8, ...x16. It is reasonable for the motherboard to specify the number of channels in the PCIe slots. The user just needed to know what the maximum width is, and buy cards accordingly.

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Open ended PCIe Slot connectors are available, on the market.
They are not quite as mechanically strong - capable of withstanding moment arm force of the add-in card leaning toward the other axis. But they can be obtained.
It is possible to manually open the end of the slot, using a small precision tool but I do not recommend trying that - usually the connector ends up cracked or split. And the shavings are likely to migrate where you don't want them to go. I thought this might work, but one failure made me realize it is not a good thing to try and DIY.
Best to look for the open ended slot connector, in the market place. However, reworking a PCIe Slot connector - removing one, and mounting a different one ... is a very tricky rework too.
So if you're building a new PCB, the slot connector had not been previously populated - this is the right time to get an open ended Slot Connector.
EDIT I saw a response from @alex.forencich above .. and that made me remember, what I should have mentioned here:
You can insert a wider ( e.g. x8, x16 etc. ) Add-In Card to a narrower ( e.g. x1, x2, x4 ) slot using a Reducing Riser. Those are available as stock items from various PCIe hardware vendors.

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