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To explain this further, the following two sentences mean different things. The short dress that is pink is my favorite. The short dress, which is pink, is my favorite. The first sentence indicates there are many short dresses, and the one that is pink is my favorite. The second sentence indicates there is only one short dress, and it happens to be pink.
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Lynn Miclea (Grammar Tips & Tools: A guide for writers and self-published authors to help with grammar, writing, and proper word usage)