Relationship between two lines that form a right angle or between two orthogonal planes.

### Properties

- The relationship on the set of lines in the plane is symmetric, but it is neither reflexive nor transitive.
- It is symmetric : if
*ℓ*\(_{1}\) ⊥*ℓ*\(_{2}\), then*ℓ*\(_{2}\) ⊥*ℓ*\(_{1}\). - It is not reflexive : a line
*ℓ*cannot be perpendicular to itself. - It is not transitive : if
*ℓ*\(_{1}\) ⊥*ℓ*\(_{2}\) and*ℓ*\(_{2}\) ⊥*ℓ*\(_{3}\), then*ℓ*\(_{1}\) //*ℓ*\(_{3}\).

### Symbol

The symbol for the relationship of perpendicularity is “⊥” which means “is perpendicular to”.

### Example

- Line
*ℓ*\(_{1}\) is perpendicular to line*ℓ*\(_{2}\).

Note that in the case of three different planes, it is possible to have \(∏_1, ∏_2\) and \(∏_3\), with \(∏_1\) ⊥ \(∏_2\), \(∏_2\) ⊥ \(∏_3\) and \(∏_1\) ⊥ \(∏_3\). However, for three lines *ℓ*_{1}, *ℓ*_{2} and *ℓ*_{3}, in the same plane, it is not possible for *ℓ*_{1} ⊥ *ℓ*_{2}, *ℓ*_{2} ⊥ *ℓ*_{3} and *ℓ*_{1} ⊥ *ℓ*_{3} at the same time.