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For questions about the Valve Hammer Editor, the tool used to create 3D maps for games running on the Goldsource engine (used in Half-Life and a few other mods and games) and games running on the Source engine (used in almost every Valve game since Half-Life 2).

Originally, Valve Hammer Editor was used to create the original Half-Life and is based off of a heavily modified version of the Quake engine. It was also used to create mods such as Counter-Strike as well as other pre-Source Valve games.

Valve Hammer Editor was later upgraded and used to create maps for the Source Engine.

https://developer.valvesoftware.com/wiki/Valve_Hammer_Editor