Sometimes a map is pasted or glued onto another material, such as cloth, to make the map more rugged and durable. Many folding maps and many wall maps were backed with cloth when issued. Maps are sometimes backed for conservation purposes, usually with thin tissue. Archival quality adhesive and backing material should be employed to prevent chemical deterioration of the paper. This protects fragile maps from further damage from handling. Maps should not be backed when there is no good reason to do so. Sometimes the words laid down or lined are used to describe the process. It is noted that some maps, particularly sea charts, were issued on a double paper, this being their normal condition.