Relationship on the set of lines in the plane or between planes that is symmetric, reflexive and transitive.

### Symbol

The symbol for the relationship of parallelism is “//”, which means “is parallel to”.

### Properties

The relationship of parallelism has the following properties :

- It is symmetric :
*ℓ*\(_{1}\) //*ℓ*\(_{1}\). - It is reflexive : if
*ℓ*\(_{1}\) //*ℓ*\(_{2}\), then*ℓ*\(_{2}\) //*ℓ*\(_{1}\). - It is transitive : if
*ℓ*\(_{1}\) //*ℓ*\(_{2}\) and*ℓ*\(_{2}\) //*ℓ*\(_{3}\), then*ℓ*\(_{1}\) //*ℓ*\(_{3}\).