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You can have a quarterback with a rifle for an arm, and you can have a running back with blazing speed and who is difficult to tackle. However, without a strong offensive line, the quarterback won’t get much of a chance to use that great arm on passing plays and the running back won’t get much past the line of scrimmage on running plays.
Conversely, a great offensive line can make even an average quarterback look great and open up gaping holes and allow an average runner to get some big gains.