One-of-a-kind prototype created during the development of the published edition. Prototypes are produced to evaluate binding materials, structural elements, and design concepts prior to final production. In most cases, the binding design differs—sometimes subtly, sometimes significantly—from the version ultimately published.

Some prototypes may share the same binding as the final production edition. The primary distinction is that all prototype books contain blank pages unless otherwise noted, and do not include an enclosure unless pictured.