Link width (x1–x16)
Beginner
How many lanes a connection uses. More lanes, more data at once.
Novice
The number of lanes in one PCIe link, written x1, x2, x4, x8 or x16. Bandwidth scales with width: a x8 link carries half as much as a x16 link of the same generation.
Expert
Negotiated at link training; a link can come up narrower than its connector (for example because of a bad lane) and run degraded. Root ports are often bifurcated, so one x16 port serves as two x8 or four x4 links.