To make it clear, juxtaposition represent a definition of placing two or more things or subjects side-by-side. In the world of art, juxtaposition is used of bringing out a certain quality, developing or creating a special effects especially when two or more different elements are used. By that concept, the audience or the viewer's attention is focused to the similarities or the differences between those elements used.

     On the other hand, juxtaposition can also be use as terms of formal elements. For example, an artist could juxtaposed an organic elements of life and nature against urban environment, as to highlight the different value among the two elements used.

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http://drawsketch.about.com/od/drawingglossary/g/juxtaposition.htm