In the case of undershot waterwheels, the water flows under the wheel in a duct (channel). The duct is made to fit the wheel. It ensures that the water cannot flow underneath or past the sides of the waterwheel without driving it. Undershot waterwheels are the oldest known type of waterwheel. The power transmission occurs via blades. At their simplest, the blades consist of a wooden board, but greater efficiencies are achieved with specially curved sheet metal blades.