The first attempts in Multi-GPU Systems were done by 3Dfx, which was bought by nVidia later. Their Voodoo video cards, which were 3D only accelerators, could be hooked up to a second Voodoo video card to gain more performance. This was called SLi. Theoretically the performance would double, practically it increased far less, depending on the video game, which is still a problem in today’s Multi-GPU Systems. Later, 3Dfx built more than one GPU on one video card. It’s latest video card, the Voodoo 6, was based on four GPUs, but it was never released. However there exist several engineering samples of the Voodoo 6, mostly owned by collectors.