The shaded areas in the picture on the left are the Kado Kami (角紙) that supports the corners of the spine. On the right are the Koyori Kami (コヨリ紙) that supports the text block itself. In this picture the twisted paper is put through two holes; some instructions are for only one hole.
Plus, an easy to watch speed version of sewing six signatures of a blank notebook: Binding Six Signatures.
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