Three points must not lie on a straight line because if they are collinear, more than one flat surface can pass through them, meaning the plane is not unique. If the three points are non-collinear, exactly one plane contains them.
Conditions: The three points are distinct.; The points are being used to define or name a specific plane.
Three points must not lie on a straight line because if they are collinear, more than one flat surface can pass through them, meaning the plane is not unique. If the three points are non-collinear, exactly one plane contains them.
Conditions: The three points are distinct.; The points are being used to define or name a specific plane.