In colloquial language, the word dime usually refers only to the ten-cent coin rather than to the quantity of money; one would not normally call two separate five-cent coins taken together a "dime". Although perhaps most Americans fail to realize it, the word is not only colloquial but official, and appears on the coin itself. The coins in the USA and Canada are similar to each other, and are physically the smallest coins currently produced by either country. While now made of sandwich-like clad layers nickel and copper, dimes were originally made of 90% silver and 10% copper, the expense of which required the coins to be small enough to prevent melt value being worth more than face value.
Dimes are important to the history of coins in that they were the first coins minted as part of the decimal system that was invented at the inception of the US monetary system.
The design and even the size of the dime has changed over the centuries. The larger sized dimes (which were also a little thinner than today's version) were minted from 1796-1828.
Draped Bust, Small Eagle 1796-1797
Draped Bust, Heraldic Eagle 1798-1807
Capped Bust (Large) 1809-1828
Capped Bust (Small) 1828-1837
Seated Liberty (various subtypes) 1837- 1891
Roosevelt (Silver) 1946-1964
Roosevelt (Clad) 1965-present
The modern dime bears Roosevelt's image partly in commemoration of his efforts for the March of Dimes campaign to fight polio from which Roosevelt suffered.