noun
seam; binding crease
Refers to the line or crease where pages are bound together, such as in a book or notebook.
この本は綴じ目が弱くなっている。
The binding of this book is getting weak.
ノートの綴じ目に沿ってページを切り取る。
Tear out the page along the binding crease of the notebook.
背 refers to the spine of a book (the outer part), while 綴じ目 refers to the inner crease where the pages are bound.
Compound of 綴じ (連用形 of 綴じる, 'to bind') and 目 ('eye; line; seam').