Canvas is a heavy woven cloth of hemp, flax or cotton. It is used for sails, tents and paintings. Canvas is typically stretched across a wooden frame called a stretcher, and may be coated with gesso before it is to be used; this is to prevent oil paint from coming into direct contact with the canvas fibres, which will eventually cause the canvas to decay. Canvas has become the most common support medium for oil painting, replacing wooden panels. Modern canvas is usually made of cotton or linen, although historically it was made from hemp. One of the most outstanding differences between modern painting techniques and those of the Flemish and Dutch Masters is in the preparation of the canvas.