Hinge

The inside portion of the flexible area where book cover meets the book spine; often used interchangeably with the term joint, which should be used to designate the outside or exterior portion of the “hinge”. A volume that has received heavy or rough use often has cracked or broken hinges.

Danish: Fals (på bogbind)
Dutch: Scharnier (-versterking)
French: Charnière
German: Gelenk (innen)
Italian: Cerniera
Spanish: Charnela, bisagra
Swedish: Fals (på bokpärm)