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Quake was a first-person shooter released in 1996. Its creator, id Software, licensed the engine to many companies, including Valve. Both GoldSrc and Source are based off of the code.
The Quake engine introduced many of the concepts still used in Source today, such as the modern variant of the BSP, the use of a PVS and PAS, visleafs and brush entities. Early GoldSrc multiplayer was based off the QuakeWorld multiplayer code.