In Bohemia, in what is now part of the Czech Republic, there is a town called Jáchymov. When the town was part of Bavaria, it was known in German as Sankt Joachimsthal or in English as Joachimsthal. The town was named for Saint Joachim who, according to some sources, was the maternal grandfather of Jesus.
In 1519, the local sovereign, Count Schlick, began striking one-ounce silver coins in Sankt Joachimsthal. These coins became known as “Joachimsthalers” and then simply “thalers.” The pronunciation changed as the term passed into other languages — dahlers in northern Germany, dalers in Dutch, dallers in English. By 1700 the accepted English pronunciation was “dollars” and the term everywhere applied to any one-ounce silver coin.
After 1497 Spain minted a silver coin called reales de a ocho or pieces of eight (they were valued at eight times the underlying coin, the real). Because the pieces of eight were similar to the thaler, they often came to be called the Spanish dollar. They were the first world currency, and were common throughout the Americas including the English colonies.
Eager still to abandon many things English, on July 6, 1785, the Continental Congress unanimously “Resolved, That the money unit of the United States be one dollar.” The following year the Congress further declared that, “The Money Unit or Dollar will contain three hundred and seventy five grains and sixty four hundredths of a Grain of fine Silver. A Dollar containing this number of Grains of fine Silver, will be worth as much as the New Spanish Dollars.” The decimal system for sub-dividing the dollar was adopted from the French on the suggestion of Thomas Jefferson.
Spanish coinage was legal tender in the United States until 1857. The pieces of eight, of course, are the source for two-bits, four-bits, six-bits, a dollar (the coins could be and were actually cut into eight pieces).
The American currency is based on Bavarian, Spanish and French precedents (and Mexican silver).