Allows standard french horn mouthpieces to be used on an alto horn. • 1-1/8 inch long (28.6mm) • 5/16 inch (7.9mm) opening (inside diameter) for horn mouthpiece. • 11/32 inch (8.7mm) shank (outside diameter) to insert into alto horn. • Tapers from 7/16 to 11/32 inches (11.1mm to 8.7mm) outside diameters. • All specifications are close approximates