Why would a PCIe x16 slot be limited to PCIe x1 card?

Why would a PCIe x16 slot be limited to PCIe x1 card? pcie x1 in x16 slot Pcie x1 slot blocked by my gpu, considering using the pcie x16 slot for my wifi card pcie 4 in pcie 3 slot For example, you can fit a PCIe x1 card in a PCIe x16 slot However, you can't do the opposite You can mount, for instance, a PCIe x1 sound

pcie 4 in pcie 3 slot Two types of PCIe slot on an Asus H81M-K motherboard Various slots on a computer motherboard, from top to bottom: PCI Express x4; PCI Express x16; PCI Express

pcie slot sizes Is the card installed in the top pci-e x16 slot? Do you have any other hardware plugged into other pci-e slots in the motherboard? All PCI Express cards will fit in the bigger X16 socket but not standard PCI cards A PCI and PCIe X16 slot is actually keyed