Factory counterweight used on straight 36, 42 and 50 series basses (yes, some bass players prefer to use a counterweight to improve their horn's balance). Useful for balancing instruments with detachable valve sections, heavy slides or other front-heavymodifications. Can be used on all 36 and 42 series tenors, as well as 50 series basses. Will not fit earlier models of smaller horns such as 6, 8, 12, 16 or 39, as their cross braces have smaller diameters (though it now appears it might work on newer builds of these horns). Designed to attach to tubes that are 3/8-inch (9.5mm) in diameter. Weight is 0.40 pounds (about 6.4 ounces or 0.18 kg). Assembly required (it's easy).