Palletizers are special machines capable of building a complete unit load on a pallet. Although the most common application is for cases, some palletizers can also handle sacks and bags. A less common palletizer application would be for special products such as sheet stock. There are two general methods employed for automatically building a full pallet. One makes use of a fixed position or overhead gantry robot with end effectors suited to the individual loads, such as vacuum lifts or a gripper. The other method employs more complex mechanisms for forming pallet layers off line, one at a time, and then shifting or transferring each successive layer onto the pallet as it is lowered via a lift/lower table.