Very early it became the practice of governments to shape and stamp
pieces of metal to be used as money, so as to indicate their weight
and fineness. The act of shaping and marking metal for this purpose is
called coinage.[5] The coinage by government had notable advantages in
giving to the monetary units uniformity of size, fineness, and value,
with the stamp that was readily recognized. But in its simplest form
coinage in no way changed the value of the money, and any other mark
equally plain put upon it would have served equally well, if only it
had carried with it equal assurance of the quality and weight of the
metal.

9. #Technical features of coinage#. For each kind of metal money there
is an established _ratio of fineness_ for the more precious material,
which is mixed with baser metals used as alloys. In the United States
all gold and silver coins are made nine-tenths fine; in Great Britain,
eleven-twelfths. The established weight of the gold dollar in the
United States is 25.8 grains of standard gold which contain 23.22
grains of fine gold. The _limit of tolerance_ is the variation either
above or below the standard weight or fineness that a coin is allowed
to have when it leaves the mint. This is different for each of the
principal coins, being about one-fifth of one per cent on a gold
eagle. The _par of exchange_ between standard coins of different
countries is the expression of the ratio of fine metal in them.
Thus the par of exchange between the American dollar and the English
sovereign (the "pound") is 4.866; that is, that number of dollars
contains the same amount of fine gold as an English gold sovereign.
The embossed design is merely to make the coins easily recognizable
and difficult to counterfeit; and milled or lettered edges are to
prevent clipping and otherwise abstracting metal from the coins.