Why English and American coins called Pound and Dollar respectively

why English coin is called pound 

In Europe,Silver coins were used commonly during the Middle Ages.The smaller silver
pennies used were 1/240 of a pound in weight,and were called sterlings.240 sterlings weighed
1 pound and the basic unit for money became the pound of silver,.the unit is still called the pound

sterling,although it is no more made of silver.

     why American coin is called dollar            

The word dollar used for American money has also a history.During the 16th century, silver coin made in St.Joachimsthal in Bohemia were thought to contain the purest

silver. this prized coin became as the Joachimsthaler which was shortened to thaler.In some

places it became Daler.In 1785 when the U.S.Congress decided to start issuing money,they

changed this to dollar.