Combinations of restriction enzymes.

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Hello

On a Gel electrophoresis why do the bands come out in one set of places if you use restriction enzymes seprately and then in in different places if you combine them? And in different numbers? For example EcoR I and Hind III. EcoR I has two restriction sites. Hind III has three restriction sites. Should not the two combined have five restriction sites? My gel only has three sites. And why are they in different places than the single lains in the gel? Thankyou