Flat or curved surfaces, inside or outside, of almost all shapes and sizes can be machined by milling. Milling processes include slab milling, side milling, end milling, slat milling, face milling, and form milling. Many kinds and sizes of cutters are needed for the large variety of work that can be done by milling. A variety of holders and drivers are needed to accommodate the many sizes and types of milling cutters. These are known as milling machine arbors, collets, and adapters. Milling machines may be classified as general-purpose, production, planer-type, and specialized machines. As in any other machining operation, planning for milling machine applications requires consideration of a number of factors, depending on the complexity of the job to be done.