What is meant by the term spine in line?

In the context of bookbinding, the term "spine in line" refers to the way in which the textblock is sewn to the cover. specifically, it refers to a style of bookbinding where the textblock is aligned with the spine of the cover. This is in contrast to "spine offset," where the textblock is offset from the spine.

Spine-in-line binding is often used for softcovers or paperback books, as it allows for a flexible spine that can accommodate the opening and closing of the book. It is also commonly used for magazines, catalogs, and other types of publications that require a durable spine.

Spine-in-line binding is a relatively simple and cost-effective method of bookbinding, and it produces books with a professional and aesthetically pleasing appearance.