Black Diamond ATC The Black Diamond ATC is a lightweight and durable belay device featuring a classic design. Thanks to its large surface area, the ATC dissipates heat when lowering or rappelling, and the smooth-handling geometry accommodates rope sizes from 7.7 mm to 11 mm. The lack Diamond ATC offers excellent belaying and rappelling performance anytime, anywhere. Black Diamond ATC Features: Unique, smooth-handling geometry. Minimalist design is durable in all conditions. Large surface area dissipates heat when lowering or rappelling. Handles ropes from 7.7 to 11 mm.