Lines that intersect at right angles.

The relationship of perpendicularity between the lines in a geometric plane is symmetric but neither reflexive nor transitive.

### Symbol

The relationship of perpendicularity between two lines is written using the symbol “\(\perp\)” and is read “is perpendicular to”.

### Example

The figure shows \(\mathit{l}_{1}\perp \mathit{l}_{2}\).

In the figure, the right angle is represented by a small square in one of the angles formed by the two perpendicular lines.

### Historical note

French mathematician Pierre Herigone was the first to use the symbol \(\perp\) as the symbol for perpendicularity in his book *Cursus Mathematicus* in 1634.