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This article is about a branch of mathematics. For the short film starring Ewan McGregor, see Solid Geometry (film).

In mathematics, **solid geometry** was the traditional name for the geometry of three-dimensional Euclidean space — for practical purposes the kind of space we live in. It was developed following the development of plane geometry. **Stereometry** deals with the measurements of volumes of various solid figures including cylinder, circular cone, truncated cone, sphere, and prisms.

The Pythagoreans had dealt with the regular solids, but the pyramid, prism, cone and cylinder were not studied until the Platonists. Eudoxus established their measurement, proving the pyramid and cone to have one-third the volume of a prism and cylinder on the same base and of the same height, and was probably the discoverer of a proof that the volume of a sphere is proportional to the cube of its radius.^{[1]}

Basic topics are:

- incidence of planes and lines
- dihedral angle and solid angle
- the cube, cuboid, parallelepiped
- the tetrahedron and other pyramids
- prisms
- octahedron, dodecahedron, icosahedron
- cones and cylinders
- the sphere
- other quadrics: spheroid, ellipsoid, paraboloid and hyperboloids.

- projective geometry of three dimensions leading to
- proof of Desargues' theorem by using an extra dimension
- further polyhedra
- descriptive geometry.

Analytic geometry and vector techniques have a major impact by allowing the systematic use of linear equations and matrix algebra; this becomes more important for higher dimensions. A major reason to study this subject is the application to computer graphics, meaning that algorithms become important.

**^***...paraphrased and taken in part from the 1911 Encyclopædia Britannica*