Rectangular is an adjective and so there is no noun in the question.
To put it another way, a rectangular what?
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It is usually a plane shape that forms a boundary of the figure..It is usually a plane shape that forms a boundary of the figure..It is usually a plane shape that forms a boundary of the figure..It is usually a plane shape that forms a boundary of the figure..
There are many plane shapes comprising 4 squares and 2 rectangles but there is no such 3-dimensional shape.
A flat shape.
cube
a face