Artifactually important volumes with damaged bindings that need to be retained should be boxed rather than treated when treatment would alter their value or character. Damaged books that have low value or are rarely used and do not warrant treatment or repair of the binding should also be boxed. Very thin, small, fragile, limp or oddly shaped books need boxes to hold them in shape, to protect them during handling, and to protect them from adjacent volumes on the shelf. Drop-spine boxes provide better support and protection from dirt and light than do phase boxes. They are now available in various styles with different types of closures.
Bindings are abraded as books slide in and out of slip cases.
In addition, the spine is left unprotected from light. It is important that a box be accurately fitted to the book. A loose fit does not provide the necessary support and allows the book to shift inside the box, exposing it to the abrasion which the box was constructed to avoid. A tight fit can cause damage to the edges and joints of the cover. The materials used to construct a box should be permanent, durable, and acid-free.