Decimalisation is the process of converting a preexisting system of currency to one based around the number ten. This process began in earnest in the 18th century and most countries in the world now base their currencies on the decimal system. The ease of doing the mathematical computations in calculating different denominations is the main reason that decimalisation gained favor. One of the most significant holdouts to the process was Great Britain, which finally converted its randomly-based pound system to the decimal system in 1971.
Most people in the modern world take a monetary system based on ones and tens for granted. In fact, the decimal system only began to grow in popularity in the late-18th century. With the United States gaining freedom from Great Britain and the French Revolution each taking place in that time period, two major international players were soon using the decimal system as the basis for their currencies. As a result, the decimalisation movement gained momentum and soon swept most of the world.
Any country that has undergone the decimalisation system usually has done so to bring its own monetary system in line with other world currencies. Although different names for the currencies can be used, the essential numerical background of each system is the same. For example, most countries in Europe use the Euro as the basis for their currencies, while the United States uses the dollar as theirs. But all of these countries use a decimal system to parcel out the different denominations.
As an example of how decimalisation works, imagine the example of the United States. The US colonists had come from a British monetary system which had a rather random method of currency allocations, such as the fact that there were 240 pence in a pound, the main British denomination. When the US first starting minting currency in 1792, they established that there would be 100 pennies in one dollar, thus cementing the decimal system for posterity.
Some countries clung hard to old currency methods before finally succumbing to decimalisation. When Great Britain finally decimalised in 1971, many citizens were upset that this was done without any popular vote. These citizens had grown used to the coinage the country had always used, and feared the change brought about by the decimal process. As a result, Decimal Day in Great Britain, which occurred on February 15, 1971, was met with skepticism, although it eventually died down as the new system became commonplace.