"Perpendicular" is a description of the relationship between two lines,
two planes, or a line and a plane.
They are perpendicular if they form right angles (90 degrees) where they cross.
-- The little lines in the 'plus' sign ( + ) are perpendicular.
-- the fllag-pole is perpendicular to the ground.
-- The wall is perpendicular to the floor.
-- The door is perpendicular to the floor.
-- The door is only perpendicular to the wall when it's wide open.
If it's only open a little bit, then it's still perpendicular to the floor,
but not to the wall.
-- Your legs are perpendicular to the floor when you're standing up.
