The PCIe specification permits slots with different physical sizes, depending on the number of lanes connected to the slot. For example, a slot with an x1 connection uses a smaller slot, saving space on the motherboard. However, bigger slots can actually have fewer lanes than their physical designation. For example, motherboards can have x16 slots connected to x8, x4, or even x1 lanes. With bigger slots, check to see if their physical sizes correspond to the number of lanes. Additionally, some slots downgrade speeds when lanes are shared. This occurs most commonly on motherboards with two or more x16 slots. Some motherboards have only 16 lanes connecting the first two x16 slots to the PCIe controller. This means that when you install a single GPU, it has the full x16 bandwidth available, but two installed GPUs each have x8 bandwidth.