A reprint may refer to a new printing of an item made from the original type image, commonly by photographic methods, or to a new edition with substantially unchanged text. A facsimile reproduction simulates the physical appearance of the original in addition to reproducing its content exactly. Reprints and facsimiles are collected when they provide useful information such as a commentary or bibliography which did not appear in previous printings, when NLM lacks the original, when the original copy in the collection is in poor condition or was issued on alkaline paper, or when they represent important scholarly initiatives or are historically significant in some way.