A primitive is by definition a very simple structure. Primitive, as it relates to the Valve Hammer Editor, refers to a brush-based object that conforms to a common shape. Primitives include arches, blocks, cylinders, spheres, spikes, tori and wedges. Some are made up of multiple brushes but are still considered primitive as they are used commonly and are of a simple shape. Two examples of multiple-brush primitives are tori and arches.
Note: Some primitives have an orientation. To set which way they face upon creation turn on the 'Reorient primitives on creation in the active 2D view' option under the Tools/Options/2D views tab. Now whichever 2D view you leave active (click in last) will be the way the object faces.
Note: Another option that may cause problems can be found under the Tools/Options/General tab: 'Stretch arches and toruses to fit original bounding rectangle.' This means any arches or tori you create will fill the box you drew. If turned off it will consider that box as defining the circle the arch or torus is drawn from.
The block, a basic cube, is the most used object when mapping in Hammer.
A cylindrical brush created with a specified number of sides.
Note: The minimum perceived valid size seems is 64x64
The sphere is a rather complicated piece of geometry, so it would be wise to be careful with the number of faces you give the sphere.
Bug: The sphere primitive in Hammer upon creation will often be reported as an invalid structure. The cause of this bug seems that the vertices are not properly positioned. Having Hammer fix the invalid structure will cause vertices to move around and will make the sphere look jagged. If you want a perfect looking sphere, you will need to use a third-party editor such as the Sledge editor, which fixes the issue, then create your sphere, save the map as a .vmf and open the map file in Hammer. Even if the brush is not reported as invalid in Hammer, you will be unable to clip the brush at all without it moving the vertex points around.
Like the cylinder, but all the top vertices come to 1 central point.
The wedge is basically a triangular prism.
Note: You are better off creating a 6-sided block and using the clipping tool.
This creates a basic arch as defined by the Arch Tool. Multiple blocks slightly rotated will be created within 1 group.
The torus resembles an inner tube or a donut. This primitive can easily mass polygons so keep the number of faces on this primitive low. Like the arch it has a special Tool.