Inlays and onlays (bookbinding)

While they are complementary techniques, and may appear similar in their final forms, they are distinct in how they are constructed.

The edge of the inlay can be tooled, in which case the edge of the inlay is beveled, with the skin side of the leather slightly larger than the flesh side.

Because of their thinness, onlays are not noticeably raised from the surface of the covering leather.

Onlays are adhered to the covering leather with paste or PVA, and their edges are usually, though not universally, tooled over in order to hide any minor irregularities.

Petraki V, Antoniadou M, Koniaris A, Sakellaropoulos O, Aivaliotis S. Polymeric onlays.