The name liner generally refers to fabric-covered moulding placed between the canvas artwork and the outer frame. It provides a visual relief between the art, the frame and the room decor.
Usually the liner is fabric-covered, but a second frame could be used to add a dash of colour, or even a gold or ornate frame to create the effect desired. Sometimes you can’t find just the right frame to encompass the piece, but by layering frames you can get the right size, colour combination or ornamentation which enhances the art perfectly.
A liner is helpful to use when you need some breathing space between a large frame and the artwork, or to keep the frame from closing in the artwork. Perhaps you want a very ornate frame on the artwork but the artwork is also quite busy, the use of a simple liner would help with the transition between the details of the art and the frame.
Liners can be used to add texture, with linen, silk or other fabrics or to contrast the texture of the canvas with smooth painted moulding. The liner should differ in width from the frame to add interest and to keep the design from being static and uninteresting.
Many times I have used a floater frame as the liner, to give a visual spacing around the canvas and then use an outer frame to finalize the design.