Slide lock. Threaded ring with finger. Attaches to upper barrel and works with your existing handslide locking lug to lock the slide in first position when not in use. Fits Bach 42, 45 and 47 series tenors and all 50 series bass trombone slides. Fits oldstyle and "New Bach" slides. Fits Stradivarius and Artisan series. May require some adjustment of the finger for maximum effectiveness. -cdp