The Post Office Department has, since its very early days, issued stamps that efficiently handle the everyday payment of postage. These stamps, whose designs might be used for many years, are known as 'definitives'. From the first U.S. issue of 1847 through the end of the nineteenth century, images on definitive stamps portrayed important political and military leaders. By the turn of the twentieth century, definitive issues incorporated common design elements, and, later, clearly-defined themes, of which the 'Presidential Issue' of 1938 provides one example. The USPS continues offering definitive stamp sets with common themes, but many definitives do not share a common subject, design, or name.