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

How To Check PCIe Version of Slot and Motherboard pcie slot length PCIe Slots · A PCIe slot that supports a full-height full-length PCIe card also supports a full-height half-length or half-height half-length pcie slot size 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

pcie 4 in pcie 3 slot ssd Riser Cards and PCIe Slots · 1-slot PCIe riser module: x16 Port0 00000 - Full-height half-length · 2-slot PCIe riser module: x8 Port12 00