Damage to hardcover boards which causes an outward or inward curve. Rapid changes in temperature and humidity levels can cause the pastedowns (pages glued to the inside boards) to expand and/or contract. When the pastedowns rapidly dry-out they also shrink, creating a downward pressure on the boards which may in turn, cause an inward warp. If the pastedowns rapidly swell in a humid environment and then stay that way for awhile, they may cause the boards to eventually curve outward.