Shepherd's whistle
A shepherd's whistle is a specialized, modulatable, variable-pitch whistle used to train and transmit commands to working dogs and other animals. Unlike other whistles, they are placed inside the mouth. The pitch is controlled by the placement of the tongue; physically, shepherd's whistles are vessel flutes with the tongue forming one side of the resonating chamber, and controlling its size. Like tin whistles, while simple, they can be used as musical instruments in their own right.