A regular letter weighing 1 ounce or less requires one "Forever" stamp or a stamp equaling the current first-class postage rate. The letter must be less than or equal to one-fourth inch thick.
A U.S. Forever stamp is always worth the equivalent of the 1-ounce first-class postage, regardless of what the rate is. For instance, someone who bought the original Forever stamp at its initial rate of 41 cents in 2007 could still use that stamp for first-class postage in 2014.
A stamp that has a monetary amount printed on it is good for that amount and can be combined with stamps of other denominations. As of October 2014, a first-class stamp for a 1-ounce letter costs 49 cents.