The answer to this question is more complicated than it may at first seem. The term "rare" is a moving target, changing meaning from country to country, institution to institution.
Here in the Rare Book & Special Collections Division at the Library of Congress, we generally consider an item to be rare if it was published prior to the year 1801. This date represents roughly the period when the Industrial Revolution transformed printing techniques, allowing for faster, cheaper production of books and other printed matter in larger quantities. Thus, materials printed after that time are less rare simply because more of them were available, and more of them remain.
Other factors can contribute to an item's "rarity" including its provenance. If an item has an interesting or important association with a famous person or group, it can also be considered "rare" regardless of the date of publication.
We hope that this is helpful; If not, please feel free to submit your question to begin a conversation with a Reference Librarian.
This portal includes some of the most interesting and important items in the collection, including a copy of the Gutenberg Bible, Thomas Jefferson’s copy of The Federalist, medieval manuscripts, books relating to cookery, children's literature, and many more.
