Grain (measure)
In many cultures, a grain is a unit of mass of the same order of magnitude as a single seed of a typical cereal crop. E.g. in Europe, the average mass of wheat or barley grains was historically used to define mass units.
Since 1958, a grain or troy grain (symbol: gr) is internationally defined in terms of the metric system by the equation
i.e. 1 grain is exactly 64.79891 milligrams.
The grain is the only unit which all three traditional English mass and weight systems (avoirdupois, Apothecaries and troy) have in common.
As a mass unit for pearls and diamonds, a metric grain or pearl grain is equal to 14 of a (metric) carat, i.e. 50 mg (0.77 gr).

# Usage in North America
Grains are currently used in the United States and Canada to measure the mass of bullets and gunpowder, and scales for handloading measure in grains; bullets are generally measured in increments of 1 grain, gunpowder in increments of 0.1 grains. Grains are used to weigh the tools involved in the sport of fencing, including the foil. Grains are used to measure arrows, and arrow parts in archery.
Grains are also used in environmental permitting to quantify particulate emissions.  Grains are used to measure the amount of moisture per cubic foot of air, a measure of absolute humidity.

# History
At least since antiquity, grains of wheat or barley were used in Mediterranean history to define units of mass; along with other seeds, especially those of the carob tree. According to a longstanding tradition, 1 carat (the mass of a carob seed) was equivalent to the weight of 4 wheat grains or 3 barleycorns. But since the weights of these seeds are highly variable, especially that of the cereals as a function of moisture, this is a convention more than an absolute law.
The history of the modern troy grain can be traced back to a royal decree in 13th century England:
By consent of the whole Realm the King's Measure was made, so that an English Penny, which is called the Sterling, round without clipping, shall weigh Thirty-two Grains of Wheat dry in the midst of the Ear; Twenty-pence make an Ounce; and Twelve Ounces make a Pound.Henry III of England
The traditional reading of this text is that it refers to the troy pound, and that the reference to sterling pennies is purely symbolic. According to a more recent reading, however, the pound in question is the Tower pound, and it talks about the actual mass of real sterling pennies.. The Tower pound, abolished in 1527, consisted of 12 ounces like the troy pound, but was 116 lighter. In any case, with both readings one needs to substitute 24 barley grains for the 32 wheat grains of the text, according to the general convention of a 4:3 equivalence, for it to make sense. The weight of the original sterling pennies was 2212 troy grains, or 24 "Tower grains" if the Tower pound was divided in the same way as the troy pound. Regardless of which pound this text originally referred to, a (troy) ounce still equals 2024 = 480(troy) grains, and a pound consists of 122024 = 5760 grains.
Originally the troy pound was only "the pound of Pence, Spices, Confections, as of Electuaries", and the merchants used different standards, which had to be compatible with those used abroad. One such standard, the avoirdupois pound, was later fixed officially at exactly 7000 troy grains. It consists of 16 avoirdupois ounces of 43712 troy grains each.