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A grain is a unit of measurement of mass, and, for the troy grain, equal to exactly 64.79891 milligrams. It is nominally based upon the mass of a single seed of a cereal.
Femtogram or femtogramme is equal to 10-15 gram (unit of mass), comes from a combination of the metric prefix femto (f) with the gram (g). Plural name is femtograms.