A motherboard with a single x16 slot will almost always have a full 16 lanes for that slot; no decisions to be made, in goes the card. If the motherboard has two physical x16s, one could be an x8 in terms of lanes and speed. If you are using multiple graphics cards, sometimes putting in both will have the 2 physical x16 slots share 16 lanes, for 8 lanes each. Some motherboards with three x16 slots will offer x16 speeds to two cards, and split one of the cards lanes if a third is added. The result would be x16, x8 and x8 speeds for the three cards.
These are only a couple examples so if you are planning to use an SLI or Crossfire setup, make sure you pay attention not only to the board’s Nvidia or AMS pedigree and number of x16 slots, but how many actual lanes are available to the slots you plan on using.
There are situations where the smaller slots function at less than maximum speeds. For instance, where there is an x4 slot and three x1 slots, the x4 slot will have four lanes. Add a card to one of the x1 slots, and the x4 slot gives up one lane to each of the x1 slots (including the empty ones), leaving only one lane for the x4. This isn’t as big a deal as it is with graphics cards, since even an x1 PCI-E slot has more bandwidth than that available to all the PCI slots combined. This is more than enough for most non-graphics expansion graphics cards that will be found in a PC.
