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

On a motherboard with PCI Express slots, each PCI Express slot is connected This allows reducing the size of the space needed on the motherboard

PCIe Base spec and various other PCIe specs show less , ECN, April 28 Unoccupied Slot Power Hand-off State Clarification This ECN allows the

pcie slot length These numbers correspond to the transmission line count in the PCIe slot The larger count of transmission lines, the larger dimensions of card

pcie slot qrelease The IO to PCIE spacing for slot '1' is standard between motherboard sizes as I understand it The rest then follow 'down' the board