The middle waist, also known as the book waist. Generally, it refers to the joint between the upper and lower book shells, that is, the position of the waist of seals one and four (the joint of seals two and three is not called the middle waist).
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Middle diameter refers to the distance between the second and third paperboards of the book cover.
Middle-diameter paperboard is also called middle-diameter paper, and refers to a cardboard strip pasted in the middle of the middle-diameter part.
Mid-seam refers to the two gaps between the middle-diameter cardboard and the book case. The middle gap left when the book case is made is used for the connection between the book case and the book block and the forming of the pressure groove. 