Cotton paper, also known as rag paper or rag stock paper, is made using cotton linters (fine fibers which stick to the cotton seeds after processing) or cotton from used cloth (rags) as the primary material. Prior to the mid-19th century, cotton paper was the main form of paper produced, with pulp paper replacing cotton paper as the main paper material during the 19th century. Although pulp paper was cheaper to produce, its quality and durability is significantly lower. Although pulp-paper quality improved significantly over the 20th century, cotton paper continues to be more durable, and consequently important documents are often printed on cotton paper. Different grades of cotton paper can be produced.