Post offices an other mail service providers typically offer a mail forwarding service to redirect mail destined for one location to another — usually for a given period of time. (In the case of the United States Postal Service's First Class Mail it is generally for a period of one year, although at least one exception has been made in the case of mail addressed to the World Trade Center.) Customers of such a service usually (but not exclusively) use mail-forwarding when they change addresses or move premises.