A connection between any two PCIe devices is known as a link, and is built up from a collection of 1 or more lanes. All devices must minimally support single-lane (x1) link. Devices may optionally support wider links composed of 2, 4, 8, 12, 16, or 32 lanes. This allows for very good compatibility in two ways:
-a PCIe card will physically fit (and work correctly) in any slot that is at least as large as it is (e.g., an ×1 sized card will work in any sized slot);
-a slot of a large physical size (e.g., ×16) can be wired electrically with fewer lanes (e.g., ×1, ×4, or ×8) as long as it provides the power and ground connections required by the larger physical slot size.
In both cases, PCIe will negotiate the highest mutually supported number of lanes.
Even though the two would be signal-compatible, it is not usually possible to place a physically larger PCIe card (e.g., a ×16 sized card) into a smaller slot —though some motherboards have open-ended PCIe slots that will allow this.