How many PCIe card sizes exist today, and where are they used?

pcie slot length   Dan pcie gen 4 in gen 3 slot

Two single-wide full-length x8 gen4 slots One single-wide half-length x4 gen3 slot preconfigured with the Apple IO card The Apple Thunderbolt

PC Talk Forum I see various mobos with, for example, 'half-length' PCIe x16 slots But this is a contradiction in terms A PCIe x16 slot by Generally speaking, there are five physical sizes of PCIe cards: x1, x4, x8, x16, and x32 (FYI, PCIe x32 does exist with a maximum of 32 lanes,

harta 138 slot The PCIe design guidelines and standards define up to 16 available lanes, which also define the size of standardized PCIe card slots  The smallest PCIe slot will run at x1 speed, while a full-size PCIe slot will run at x16 PCIe x16 slots are generally intended for use with Graphics Cards