Typical PCI cards have either one or two key notches, depending on their signaling voltage.
Cards requiring 3.3 volts have a notch 56.21 mm from the card backplate; those requiring 5 volts have a notch 104.47 mm from the backplate.
This allows cards to be fitted only into slots with a voltage they support. "
Universal cards" accepting either voltage have both key notches.
Typical PCI cards have either one or two key notches, depending on their signaling voltage. Cards requiring 3.3 volts have a notch 56.21 mm from the card backplate; those requiring 5 volts have a notch 104.47 mm from the backplate. This allows cards to be fitted only into slots with a voltage they support. "Universal cards" accepting either voltage have both key notches.
Peripheral Component Interconnect