The term "linen mounted" or linen backed refers to a 19th century French process of placing paper on fabric. Today the posters are placed on duck canvas, but the obsolete term is still used to indicate a mounted poster. Linen mounting is the preferred method of restoring and maintaining posters. Using all natural substances, the posters are
laid on the canvas (with a layer of Japan paper between the two surfaces.) They are de-acidified and any minor restoration work is done. Linen mounting increases the value of a poster.
(Text Source: The Nostalgia Factory)