When the stitches are not broken or loosened, and still hold sections
firmly together.
When the joints are loose.
When the book has once been rebound.
The tendency among librarians is to mend too much before rebinding.
Mending makes rebinding more difficult and jeopardizes the life of the
book.
It is only in rare cases, as for example when a book is out of print,
and yet valuable to the library, that a second rebinding is justified.