Perpendicular
Two lines are perpendicular if they cross at right angles. You can test this by extending the lines until they cross (if they do not cross then they are parallel). Where the two lines cross, draw a right triangle, and then see whether the triangle conforms to the Pythagorean Theorem - if so, then your lines are crossing at a right angle, and they are perpendicular.
If Line A and Line B are both perpendicular to Line C, then Line A and Line B have to be parallel to each other.
All right triangles are made from two perpendicular line segments. The corners of rectangles (and squares) are also made of perpendicular line segments.
Two planes can also intersect at right angles, and then they are perpendicular planes. The sides of a cube are perpendicular planes.
Parallel
Rectangles
Squares
Circles
Triangles
Main Geometry page
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