For geometric figures in a plane, two straight lines must either be parallel to one another or must intersect at one point. Skew lines are non-parallel and do not intersect. Skew lines must therefore lie in separate planes from one another. Since skew lines are defined in terms of distinct planes, discussing such lines leads directly to the branch of mathematics called solid geometry.

Solid geometry is the branch of Euclidian geometry (named for Euclid, c. 325 B.C.E.–265 B.C.E.) that examines the relative positions, sizes, shapes, and other aspects of geometric figures that are not in a single plane. Whereas plane geometry is about two-dimensional space described by parameters such as length and width, solid geometry concerns itself with three-dimensional space.

One example of a three-dimensional object is a cube, which has height, length, and width. Another familiar example of a solid (three-dimensional...