Other than the $2 coin (for which there is no United States analog), the denominations of Canadian coinage correspond to those of United States coinage. The sizes of the coins other than the 50¢ piece are roughly equal to those of U.S. coins, though this was not always the case. They have a different metallic composition and most of them are thinner, and thus weigh slightly less, than the analogous U.S. coins. The U.S. penny settled on its current size in 1857, whereas the Canadian penny was much larger () until 1920. Because they are easily mistaken for each other and the difference in their official value is generally small, modest quantities of U.S. and Canadian coins circulate at par in the other country, particularly near their shared border. Their differing physical characteristics prevent them from being accepted interchangeably by most coin-operated machines.