A straight **r** is perpendicular to a plane if and only if** r** is perpendicular to all lines of passing through the intersection point of **r** and .

Note that:

if a straight

**r**is perpendicular to a plane then it is perpendicular or orthogonal to the entire straight line of :

- so that a straight
**r**be perpendicular to a plane , it is sufficient to be perpendicular to two competing straights, contained in :

Notice in the picture below why it is not enough that **r **be perpendicular to a single line **t **in so that it is perpendicular to the plane: