"PCI Express's serial point-to-point nature requires few sideband signals,
meaning the the bandwidth per pin can be very high in comparison to PCI
Conventional. The upshot of this is that a x1 connector can be a fraction of
the size of a PCI Conventional slot, whilst still providing twice the
bandwidth on its first implementation. Bandwidth can be further scaled up by
increasing the number of lanes running two and from a device and the PCI
Express standard allows for x1, x4, x8 and x16 slots."