The PCI Express bus defines a different type of slot based on the number of lanes in the system. For example, the physical size of the PCI Express bus x1 slot is different from that of the PCI Express bus x4. In Figure 4, you can see the difference between the PCI Express bus slots.

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Figure 4: Types of PCI Express slots.

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Figure 5: Detail of the PCI and PCI Express slots in the motherboard.

The PCI Express x16 slot was developed to be used by video cards.

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Figure 6: Difference in the edge contacts of PCI Express, AGP and PCI video cards.