NVIDIA's nForce 4 SLI Chipset
The PCI Express standard in theory allows an infinite number of parallel PCI Express lanes, but the new consumer chipsets are limited to 20 or fewer lanes. As a single PEG slot (PCI Express for graphics) already takes up 16 lanes, that leaves a meager four for other slots.
NVIDIA thought up a trick for the nForce 4 chipset . When operating a graphics card, all 16 lanes are available to the slot. In SLI operation however the 16 lanes are simply distributed among the two x16 slots. So the PEG slots are only operated with x8 PCIe.
In practice that has no negative effect on performance, as the x8 PCIe bandwidth still corresponds to AGP 8x performance. Internal comparison tests between x8 and x16 PCIe operation displayed no difference in performance.