A PCIe card fits into a slot of its physical size or larger (maximum ×16), but may not fit into a smaller PCIe slot (×16 in a ×8 slot). Some slots use open-ended sockets to permit physically longer cards and negotiate the best available electrical connection. The number of lanes actually connected to a slot may also be less than the number supported by the physical slot size.
An example is a ×8 slot that actually only runs at ×1. These slots allow any ×1, ×2, ×4 or ×8 card, though only running at ×1 speed. This type of socket is called a ×8 (×1 mode) slot, meaning it physically accepts up to ×8 cards but only runs at ×1 speed. This is also sometimes specified as "×size (@×capacity)" (for example, "×16(@×8)"). The advantage is that it can accommodate a larger range of PCIe cards without requiring motherboard hardware to support the full transfer rate. This keeps design and implementation costs down.