One price covers postage, and the other goes toward the special event. Two examples are the Chicago Century of Progress sheets issued by the US in August 1933. A common and useful way of selling stamps, that remains in practice due to the continuing oldfashioned integrity of dealers and collectors. These stamps, in the view of many collectors, are not as desirable in a collection, and usually fetch much lower prices when sold. Perforated on the top and bottom, not perforated on the sides.
Many collectors specialize in first day covers.
The US released only imperforate stamps until 1857, and very few thereafter. The stamps often make up the parts of a single picture. An important measure of a stamp's quality is the centering of its vignette. A well centered stamp has an even, unbroken edge around its vignette. A clean cancellation.