Precanceled stamps represent one kind of overprinting. Whenever an overprint changes the value, it is called a surcharge. A watermark is a special design or pattern pressed in to the soft paper during the manufacturing process. These impressions take many forms, such as letters, animals, fruit, emblems, and combinations of these and others. They are frequently hard to find under normal inspection.
Watermarks help provide protection against forgery.
One denomination is the amount of postage that stamp pays for and the other, usually smaller in size than the first, is the amount that will go to support charitable organizations or help pay for health programs. The United States has issued only one semipostal stamp to support breast cancer research in 1998. Perforations should be equal distances from the printed area. When they are, the stamp is well centered. When they are not, the stamp is off-center.