Liberty Bell stamps issued by the U.S. Postal Service beginning in 2007 can still be used today. The Liberty Bell stamps are part of the Forever stamp collection, which increase in value as the postage rate increases, and are always considered sufficient postage for mailing a single letter.
The Postal Service chose the Liberty Bell design for its Forever stamps because it is unchanging. These Forever stamps are intended to eventually replace stamps set to a specific denomination. The Postal Service honors old stamps, although they may decrease in utility as the postage rate changes. Stamps printed with a specific monetary value must be supplemented by additional stamps when postage rates increase.