Brushes are used in some 3D

3D computer graphics

3D computer graphics are graphics that use a three-dimensional representation of geometric data that is stored in the computer for the purposes of performing calculations and rendering 2D images...

 video games such as games based on the Source engine

Game engine

A game engine is a system designed for the creation and development of video games. There are many game engines that are designed to work on video game consoles and personal computers...

, its predecessor the Goldsrc

GoldSrc

GoldSrc, or Goldsource, is the retronym used internally by Valve Software to refer to the heavily modified Quake engine that powers their science fiction first-person shooter Half-Life ....

 engine, Unreal Engine

Unreal Engine

The Unreal Engine is a game engine developed by Epic Games, first illustrated in the 1998 first-person shooter game Unreal. Although primarily developed for first-person shooters, it has been successfully used in a variety of other genres, including stealth, MMORPGs and RPGs...

's tool Unreal Editor

UnrealEd

UnrealEd is the level editor used to create levels for Unreal. It was also used for other games based on the Unreal engine, such as Deus Ex and Lineage II, although it has changed along with the engine for later games...

, etc. to construct levels

Level (video gaming)

A level, map, area, or world in a video game is the total space available to the player during the course of completing a discrete objective...

. Brushes can be primitive shapes (such as cubes, spheres & cones), pre-defined shapes (such as staircases), or custom shapes (such as prisms and other polyhedra). Using CSG operations

Constructive solid geometry

Constructive solid geometry is a technique used in solid modeling. Constructive solid geometry allows a modeler to create a complex surface or object by using Boolean operators to combine objects...

, complex rooms and objects can be created by adding, subtracting and intersecting brushes to and from one another.