What does electrical x16 or x8 or x4 mean?

Question

Given these Expansion Capabilities (Specifically for PCIe) for a motherboard 975x:


One Primary PCI Express* x16 (electrical x16 or x8) bus add-in card connector
One Secondary PCI Express* x16 (electrical x8) bus add-in card connector
One PCI Express* x16 (electrical x4) bus add-in card connector

I understand that x for PCIe means: 100 / 200 MB/s of bandwidth. and 4 , 8 , 16 multiplies this amount.

But what does "Electrical x16 and x8 ..." in parentheses mean?

I want to use the full potential of the motherboard capabilities for graphics. If you just tell me what does "Electrical x16 and x8 ..." means, that might be helpful

Thanks

Answer 1

x16 PCIe slot is a standard slot for maximum of 16 PCIe lanes. "x16 electrical" means all lanes are active or fully functional. "x8 electrical" means only 8 of the 16 lanes are functional. "x4 electrical" means only 4 lanes are functional. High end cards like the 8800 series will be bottlenecked by the x8 and x4 (electrical) slots.

Answer 2

Elecrical determines the physical size and bandwidth of the PCIe slot. Check http://en.wikipedia.org/wiki/PCI_Express for more information.