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Moiré patterns result from superposition of two gratings. The uniform grating is the reference grating and the non-uniform is the deformed or specimen grating. The moire pattern thus shows the difference between the two patterns. This helps among other things to identify the difference between the two gratings. Subtle differences can be highlighted as in this animation. Shifting the reference grating moves the moire pattern. If the moire pattern moves in the same direction as the grating shift, this implies that the deformed grating has a larger spacing between the lines than the specimen grating. In this example it helps differentiate between hills and valleys.