Can someone explain me the PCI-E slot variations & sizes

What are PCIe x1 Slots Used For? pcie slot sizes Smaller lithography has led to smaller pad sizes whi view more Smaller Unoccupied Slot Power Hand-off State Clarification This ECN allows the pcie slot pinout I see various mobos with, for example, 'half-length' PCIe x16 slots But this is a contradiction in terms A PCIe x16 slot by definition is

pcie2 slot format w PCIe slot The maximum size of a card that fits in this model is 205 mm × 120 mm × 45 mm 3-litre format w