The preservation of non-book materials in a library requires a carefully controlled physical environment and a suitable circulation policy. Non-book materials include photographs, maps, manuscripts, audio and video recordings, films, slides, microforms, compact discs, DVDs, posters, charts, and other special collections. These materials are often more sensitive than ordinary books and therefore require special storage and handling conditions. The physical environment should be clean, stable, and free from harmful environmental factors. Temperature and relative humidity should be controlled according to the nature of the material. Generally, a moderate and stable temperature and relative humidity are preferable, while sudden fluctuations should be avoided. Photographic materials, films, magnetic tapes, and microforms may require cooler and more controlled conditions than ordinary library materials.