Coplanar lines that share all of their points or none of their points.
- The parallel relationship between the lines in the geometric space is reflexive, symmetric and transitive.
- The distance between two parallel lines is constant.
- A parallel relationship is an equivalence relationship, as it is reflexive, symmetric and transitive.
The symbol for parallel lines (//) was first used by William Oughtred (1574-1660) in his work Opuscula Mathematica Hactenus Inedita, which was published posthumously in 1677.