Since most papers are dried to a moisture content of about 3% (by weight of the paper), they tend subsequently to absorb moisture until the moisture content amounts to approximately 5 to 9% (depending upon the humidity of the atmosphere in which they are stored). This tendency is increased once the paper is cut, because cut edges are able to absorb atmospheric moisture faster than the surface of the sheet itself, especially when the paper is stacked in piles. When a stack of paper absorbs moisture, the edges, particularly those corresponding to the cross direction, will expand more rapidly than the center, which causes cockling. In like manner, a "spongy" effect may occur, due to absorption of moisture by the top surface of the upper sheets in the pile. This expansion may take place between the printings of a multi-color printing sequence, or when the paper is dampened as in the offset-litho printing process, with the result that the colors used will overlap in some places and leave gaps in others. Moisture absorption, particularly when the grain direction of the paper does not run parallel to the spine of the book, can cause cockling along the binding edge and waviness in the leaves, resulting in a book that will not close properly. The latter problem can sometimes be overcome, at least to a degree, by pressing the book in an atmosphere of low relative humidity, but the problem of cockling along the binding edge can only be overcome by a method ofIMPOSITION which results in the grain direction of the paper running parallel to the binding edge. (17 , 42 , 144 , 156 , 276 )