What is a Troy Pound?
When someone speaks of something weighing a pound in modern times, they are most likely referring to an Avoirdupois pound - equivalent to 7000 grains, 16 "avoirdupois" (i.e. ordinary / modern) ounces or 453.59237 grams. However in ancient times, there were several different pounds owing to the difficulty in standardization of weights across wide areas and different kingdoms.
Variant pounds evolved in different regions and for different purposes, with inevitable difficulties caused by the discrepancies. The Troy pound was originally one of these pounds and was believed to originate from the Fair of Troyes, in Champaign, France - one of the largest and most important fairs of medieval times. It is a completely different weight to the Avoirdupois pound, which came later. The Troy pound weighs 5760 grains and contains twelve troy ounces. A troy ounce is 31.1034768 grams (373.2417216 grams in total).
In order for trade to work well, a standardized system of weights was important, and the pound of Troyes happened to be the weight chosen by King Henry II (ruler from 1154-1189) as the English standard. This may have been partly happenstance, due perhaps to the fact that Henry II of England had substantial French holdings, being descended from French royal lineage. The Troy pound system was a success and its use persisted for several hundred years.
In the pre-decimal coinage system introduced by King Henry II, the Troy pound was divided into Troy ounces, shillings, pennyweights and grains. The Troy pound contained twenty shillings of twelve silver pennies each, or 240 pennyweights (abbreviated to dwts) of silver in total. Each pennyweight contained 24 "grains" of silver; the grain coming around as a standardized weight due to the average weights of actual cereal grains. The silver used at that time was 0.925 purity, and this was created by alloying 222 pennyweights (dwts) of silver with 18 dwts of base metal.
Debasement and other tricks
In the time of Henry II, the pound of money and the pound of silver were one and the same - there was no separation between the monetary value of the coin and the value of the precious metal it contained. This system was very elegant in that it prevented inflation; with goods staying the same price for generations as monetary value and the value of tangible commodities were tightly bound.
However the system was prone to abuses of other kinds, including debasement (the creation of coins of lesser purity in order to make an amount of silver "go further"). With debasement, a King or moneyer could create more coins from the same amount of silver or gold. This practice appears to have been rife with medieval moneyers, some of whom covertly minted coins of low purity - and certain gold coins of the medieval era are noticeably paler in hue than others. Debasement was even employed overtly by certain monarchs, notably Henry VIII. The debasement of Henry VIII caused inflation, was considered oppressive to the poor and was highly unpopular with the British people.
Another widespread practice was the shaving or "clipping" of silver coins by those whose hands they passed through, in order to gather extra silver which could then be sold. There was even a sly practice called "sweating", where a large bag of coins was agitated for a period of time and the silver dust collected. The shaving of coins is sometimes considered a form of debasement, as it lowers the amount of precious metal in the coin. It was difficult to prevent, and there was always the excuse that the clipper was suspicious that the coin was counterfeit, and needed to check to make sure the coin was not made of some base metal underneath! The addition of the "long cross" to the design of certain coins such as the silver penny was one of the methods used to limit the removal of silver from the coin's edges - however even this was limited in its effectiveness. Eventually the rise of "milled coinage" and machine-made coins with edge markings put an end to the practice. Another part of the strategy employed was the introduction of the Britannia silver (.958 purity) standard for silver plate, in order to reduce the outlet for scrap 925 silver and thus discourage the melting of coin shavings. 
Another debasement tactic employed by monarchs was the shrinking of the size of the actual coins, rather than the diminishing of their purity: In Henry II's time, the Troy pound was divided into 20 shillings of money and of weight; by the time of the "Great recoinage" of 1816, the Troy pound of 925 silver was divided into 66 shillings weight of 925 silver, yet only 20 shillings of coin made up a pound of money! In other words, one pound of 925 silver was used to make over three pounds of money. The currency was gradually devalued over time; and while some have blamed one group or another for the problem, closer inspection shows that people from all walks of life "had their hand in the cookie jar" and that dishonesty is a temptation that crosses nationality, social class and profession alike.
The Troy pound has now been abolished and since 1879 is no longer a legal unit of trade in the UK, Canada, Australia and other locations - although the Troy ounce is still in use for the measurement of precious metals.
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