A functionally classified barn is a barn whose style is best classified by its function. Barns that do not fall into one of the broader categories of barn styles, such as English barns or crib barns, can best be classified by some combination of two factors, region and usage. Examples of barns classified by function occur worldwide and include apple barn, rice barn, potato barn, hop barn, tobacco barn, cattle barn (pole barn), and the tractor barn. In addition, some barns incorporate their region into their style classification. Examples include the Wisconsin dairy barn, Pennsylvania bank barn, or the Midwest feeder barn. Classifications Tobacco barns Tobacco barns were once an essential ingredient in the process of air curing tobacco. In the 21st century they are fast disappearing from the American landscape in places where they were once ubiquitous. U.S. States, such as Maryland, have sponsored programs which discourage the cultivation of tobacco. In 2001 Maryland's state sponsored program offered cash payments as buyouts to tobacco farmers. A majority of the farmers took the buyout and hundreds of historic tobacco barns were rendered instantly obsolete. As tobacco barns disappear farmer have been forced to change their methods for curing the crop. In Kentucky, instead of curing tobacco attached to laths in vented tobacco barns as they once did, farmers are increasingly curing tobacco on "scaffolds" in the fields. Design Design elements which were common to American tobacco barns include: gabled roofs, frame construction, and some system of ventilation. The venting can appear in different incarnations but commonly hinges would be attached to some of the cladding boards, so that they could be opened. Often the venting system would be more elaborate, including a roof ventilation system. The interior would have its framing set up in bents about ten to fifteen feet apart so that laths with tobacco attached to them could be hung for drying. There is no one design that typifies tobacco barns but they share some common elements not seen in other barns. However, tobacco barns do cross over into other barn styles of their day. Some common types of barn designs integrated into tobacco barns include, English barns and bank barns. Hop barns Also known as hop houses or hop kilns, hop barns were very common in areas of the United States where hops were grown. Hop barns were so common it was said that "every other farm"