Why were medieval library books chained to shelves?

Short answer: some medieval and early-modern libraries chained valuable books to desks, lecterns or shelving because manuscripts and early printed volumes were expensive, scarce resources. The chain was long enough for a reader to open and consult the book nearby but short enough to stop the volume leaving the room.

Security without locking every book away

Medieval libraries often belonged to monasteries, cathedrals or universities. Hand-copied manuscripts represented large investments of skilled labor and materials. Chaining offered a compromise between two needs: protect a valuable object and still let scholars use it.

The University of Chicago Library notes that by the early 1300s some libraries had browsable reference collections attached by chains to reading desks. Oriel College records distinguish between books that could circulate and a chained reference collection, which shows why “all medieval books were chained” is another overstatement.

Why were some books shelved with the spine facing inward?

The hardware helps explain the strange orientation seen in surviving chained libraries. Chains were commonly fixed to a cover or board rather than the spine, because repeatedly pulling from the spine could damage the binding. Shelving a volume with its fore-edge facing outward let the chain run more directly to a rod or desk and reduced tangling when the book was opened.

Printing did not end the practice overnight

As printed books became more plentiful, replacement became easier and library systems changed. Chaining gradually declined, but it survived surprisingly late in some institutions. Oriel College, for example, records chained books into the 18th century.