Definition: Lying in the same plane
Try this Drag the blue rectangle, or ABCD, PQRS.
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Two objects are coplanar if they both lie in the same plane.
In the figure above, the two rectangles ABCD and PQRS are both coplanar.
Try dragging them about and you will see they both lie in the yellow plane P.
The blue rectangle however is in a different plane, and is therefore not coplanar with ABCD and PQRS.
Drag it around and see that it lies in a plane perpendicular to plane P.
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