AGP video cards working at 1x or 2x modes are fed with 3.3V, while x4 and x8 video cards are fed with 1.5V. The first motherboards with AGP slots allowed only the installation of AGP video cards fed with 3.3V. If you installed a 1.5 V AGP board in one of those old motherboards, which only had 3.3V output, both the video card and the motherboard could burn! To avoid such type of problem, the AGP bus specification defined three types of slots: one, called Universal, allows both 1.5V and 3.3V boards to be installed; another one that only allows 3.3 V boards to be installed; and another one that should only be used by 1.5 V video cards.
In August, 1998 a new specification of the AGP bus was launched: AGP Pro. AGP Pro defined a larger slot, with more voltage pins, for high-consumption 3D video cards. The AGP Pro slot is compatible with the previous versions of the AGP bus, in other words, you can install conventional 1.5 V or 3.3 V AGP video cards in AGP Pro slots.
Figure 3: Types of AGP slots.
Figure 4: 1.5 V AGP card being installed in a 1.5 V AGP slot.
Figure 5: It is not possible to install a 3.3 V video card in a 1.5 V AGP slot.