Also called basic sheet size, basic size is the size of a sheet of a particular grade of paper at which its basis weight is determined. The basic size varies according to paper grade: for book paper it is 25 x 38, for bond paper it is 17 x 22, for cover paper it is 20 x 26, for bristol paper it is 22H x 28H, for index paper it is 25H x 30H, and for tag paper it is 24 x 36. Basis weight, then, is based on the above sizes for each of those grades. (See Basis Weight.) In the metric system, used primarily outside the United States, the term grammage is used instead of basis weight. Grammage is simply the weight of the paper in grams per square meters of area. Grammage is independent of basic size, but there are also basic sizes in the metric system. (See A Series and B Series.)