Piedforts

Piefort, or more properly, Piedfort, means literally any coin struck on an unusually thick planchet as a trial piece or essay.  The designation is applied chiefly to coins of Bohemia, the Low Countries, and France, where some of these pieces were undoubtedly used as current money.  The Dickgroschen of Prague are so termed, and in the French series Pieforts of billon occur as early as the reign of Louis VII (1137-1180), while those of silver and gold from the fourteenth to the seventeenth centuries are frequently met with.
 
 
 
 

A coin struck with ordinary dies on an unusually thick planchet.  It is not intended for circulation... Pieforts have been minted since the late thirteenth century; France, Germany, Bohemia, and to a lesser degree England have been centers of their production.  Their original purpose is unclear. While some may have circulated, they do not seem to have been primarily designed to do so.  Their weights bear no logical relationship to other coins.  It seems .. pieforts were not originally intended as circulating multiples of regular coins.

What was their purpose?  L.A. Lawrence, the noted numismatist, convincingly argued that English pieforts at least were designed as models for circulating coins: a master engraver would create two dies as prototypes, and use them to strike pieforts.... Lawrence's theory is a sound explanation for the coins' thickness: no one at a mint would confuse them with ordinary coins.

In modern times the piefort has taken on other characteristics and meanings.  Its modern uses emphasize its role as a pattern, with medallic overtones.  An extreme example of this is a pattern twenty-dollar American gold piece of 1907, which was struck on a planchet the size of those used for ten-dollar pieces, but twice as thick.
 

Frey, Albert R., Dictionary of Numismatic Names, Barnes & Noble, Inc, 1947

Doty, Richard G.,  The Macmillan Encyclopedic Dictionary of Numismatics, Macmillan Publishing Co. Inc., New York, NY 1982