A distortion of paper characterized by a lack of flatness. Curl can have a number of different causes: differences in structure between a paper's wire side and felt side cause each side to expand and contract in different ways when moisture is gained and lost, producing a curl; paper that is wrapped very near the core of a paper roll and then sheeted, will have a curl, if it has not been decurled properly (called roll-set curl); dimensional changes caused by the addition of moisture to a paper's surface during offset printing causes a curl. Curl is usually more apparent in lower-basis weight papers. Curl can possibly be counteracted by running the paper through a press with a blank plate and merely applying moisture to the side of the sheet opposite the curl. Failing that, two-sided offset print jobs typically print better if the first side is printed curl down. Adding moisture to this side during printing may help neutralize the curl.