A frustum is the portion of a solid – normally a cone or pyramid – which lies between two parallel planes cutting the solid. Degenerate cases are obtained for finite solids by cutting with a single plane only.
The formula for the volume of the frustum is 
,where h is the height from the top base to the bottom base, B1 is the bottom base, and B2 is the top base.
An example of a pyramidal frustum may be seen on the reverse of the Great Seal of the United States, as on the back of the U.S. one dollar bill. The "unfinished pyramid" is surmounted by the "eye of providence".
Certain ancient Native American mounds also take on the frustum of a pyramid.
The focal field of a still or video camera forms a frustum. In 3D computer graphics, this is called the viewing frustum.
The spelling frustrum, although strictly an error, is becoming frequent enough to be considered a variant.
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