Even though it was still not using true 3D, id Tech 1, first used in "Doom" (1993) and again from id Software, removed these limitations. It also first introduced the concept of binary space partitioning (BSP). Another breakthrough was the introduction of multiplayer abilities in the engine. However, because it was still using 2.5D, it was impossible to look up and down properly in Doom, and all Doom levels were actually two-dimensional. Due to the lack of a z-axis, the engine did not allow for room-over-room support.