This video shows what a fillet looks like inside a mat. A fillet is a small wood frame strip that attaches either to the perimeter of a window on a mat, or under the lip of a wood picture frame.
When a fillet is paired with a mat, we build up the back with foam core so that the back is flush. This allows the material to have even pressure and lay flat as it rests against your art.
The fillet comes completely adhered to the mat. We tape or glue the materials so that all you have to do is attach your art.