Because paper is composed of a randomly felted layer of fiber, it follows that the structure has a varying degree of porosity. Thus, the ability of fluids, both liquid and gaseous, to penetrate the structure of paper becomes a property that is both highly significant to the use of paper. The need to limit the spreading of ink resulted in "sizing" the paper with gelatinous vegetable materials which had the effect of sealing or filling the surface pores. Later, the term "sizing" was applied to the treatment of paper stock prior to the formation of the sheet, with water-repellent materials such as rosin or wax.

Resistance towards the penetration of aqueous solution / water is measured by Sizing or Cobb values.