Markwort The Perfect Circle Pitcher's Training Aid, 13 - 17-Inch Circumference When practicing windmill pitches for softball, the arm should touch the flap every time Excellent pitching aid for basic mechanics or advanced pitching machines such as fast ball, change-up, drop ball, screw ball, rise ball, and curve ball Fits in your pocket Includes instructional CD and carrying case New larger size is 15"-17" in circumference.