parallel lines and parallel planes

Straight lines or planes that always remain a constant distance from one another no matter how far they are extended.

Examples of parallel lines found in everyday life include railway lines, the verticals of goal posts, and shelves in a book-case. Parallel lines are shown in diagrams by using single or double arrows:

The definition of parallel lines as lines remaining a constant distance apart is a principle of Euclidean geometry. Some non-Euclidean geometries, such as elliptical and hyperbolic geometry, however, reject Euclid's principle.