The AGP card communicates with the AGP motherboard using a ‘signalling voltage’ that varies according to the version. AGP 1.0 uses 3.3 volts, AGP 2.0 uses 1.5 volts, and AGP 3.0 uses 0.8 volts. Depending on the AGP version supported, the AGP card has one or two slots. 3.3 volts uses one type of slot, while 1.5 and 0.8 volts use the same type of slot.
For the AGP card to work with the motherboard both the slot type and the signaling voltage has to match.