Lines that are not in the same plane and that do not intersect.

Noncoplanar lines.

Lines that are in two different planes

(n) Nonintersecting, nonparallel lines in 3-D space. The relationship of skew lines, such as the shortest distance (clearance), can be calculated using descriptive geometry techniques.

In geometry, skew lines are two lines in Euclidean space that do not intersect but are not parallel. A simple example of a pair of skew lines is the pair of lines through opposite edges of a regular tetrahedron. Lines that are coplanar either intersect or are parallel, so skew lines exist only in three or more dimensions.