Let's consider the following situations:

**a) line contained in the plane**: if a straight **r** has two distinct points on a plane , then **r** is contained in this plan:

**b) competing line or incident to the plan**: we say that the straight line **r** "stick" the plane or that **r** and are competitors in **P** When .

Note: the straight **r** is reverse to all straight lines that do not pass the point **P**.

**c) line parallel to the plane**: if a straight **r** and a plan have no point in common so the straight line **r** is parallel to a straight line **t** contained in the plan ; therefore r // .

In there are infinite parallel, reverse or orthogonal lines to **r**.

P_{11}) If two distinct planes have a point in common, then their intersection is given by a single line passing through that point.