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In the United States the customary or commercial term for expressing the "weight" per unit area (more properly "mass per unit area") of paper has been "basis weight," "ream weight," or "substance." These are defined as the mass in pounds of a ream of a given sheet size and number of sheets (usually 500 sheets, occasionally 480 sheets). In most other countries the mass per unit area is expressed in grams per square meter, g/m². The French term for mass per unit area, "grammage," is recommended by ISO Committee TC 6 on Paper for use in English as well as in French because of its convenience and clear relationship to g/m².

The mass per unit area of paperboard has been expressed in the customary system as the mass in pounds per thousand square feet, and in the metric system as grams per square meter (g/m²).

The SI metric units, in which grammage (mass per unit area) is expressed in g/m², are the preferred units for TAPPI Test Methods.