The is one of the following closely-related belay tubes, all called "ATCs" after the original Black Diamond Air Traffic Controller:

Image Device Shell Style Keeper Advanced Base Camp Advanced Rope Controller, Version A I 3.8 mm. Advanced Base Camp Advanced Rope Controller, Version B I 4.7 mm. Advanced Base Camp Advanced Rope Controller, Rigid III 4.7 mm. AustriAlpin, Version A I 3.8 mm. AustriAlpin, Version B III 3.8 mm. Black Diamond Air Traffic Controller, Version B I 3.0 mm. Black Diamond Air Traffic Controller, Version B I 3.3 mm. Black Diamond Air Traffic Controller, Version C I 3.8 mm. Black Diamond Air Traffic Controller, Version D I 3.8 mm.	Image Device Shell Style Keeper Black Diamond Air Traffic Controller, Version E IV 4.7 mm. Black Diamond Air Traffic Controller, Version F IV 4.7 mm. Brasovia Wedgemount II 4.0 mm. Camp, Version B II 4.0 mm. Camp Shell, Version A I 3.8 mm. Camp Shell, Version B II 3.8 mm. Climb X Max-Air II 4.0 mm. Climbing Technology Double III 3.8 mm. Cypher Arc, Version A II 4.0 mm.	Image Device Shell Style Keeper Cypher Arc, Version B III 5.2 mm. Fusion I 3.8 mm. Good Makings II 4.0 mm. GrandWall Wedgemount II 4.0 mm. Singing Rock I 3.8 mm. Stubai, Version A I 3.8 mm. Stubai BC III 3.8 mm. Zero-G G-Spot I 3.8 mm.

Each of these is 54±3 mm. long, 46±1 mm. wide, and weighs 59±9 g. Their slots are all 32±1 mm. long and 14±1 mm. wide. They all have plastic-covered cable keepers.

The Black Diamond ATC was the first of these to appear, by many years. The basic design evolved and similar devices appeared from others.

The ATC is a lightweight, popular belaying device among climbers. Rigging is simple: insert a bight of rope and clip it with a suitably anchored carabiner, making sure that the rope is not running over the keeper. Two-rope rigging is similar. On thinner ropes, adding another carabiner helps.

My biggest complaint is that none of these give me enough friction when rappelling with a heavy load on fast 9 mm. rope. I also prefer more friction while belaying, unless my partner is particularly lightweight. For these reasons, I prefer using a Trango Jaws or one of its equivalents.

Any of these will overheat badly on long rappels. Overheating is not an issue when belaying, but can be a concern when lowering someone more than a short distance.

There are four different shell styles in use::

1. There is a small shelf at each end of the shell with a hole passing through it. The keeper cable has a crimp at the ends, which are visible from the top of the device.
2. There is a small shelf at each end of the shell with a blind hole in the bottom. The keeper cable enters the holes, and is staked in place. The stake dimples are visible at the ends of the device.
3. There is a bulge at each end of the shell with a blind hole in the bottom. The keeper cable enters the holes, and is staked in place. The stake dimples are visible at the ends of the device.
4. Like style III, but the shell has a shallow valley above the bulge.

The differences between the styles are cosmetic rather than functional.

None of the keepers-to-shell are strong enough to support body weight, so don't be stupid enough to rely on a keeper to protect you.

There are also several different diameter keeper cables used on these. Cable keepers are a compromise between a cord that stows easily but tends to get tangled in use, and a rigid rod that stands up to the rope running over it (by accident, of course). I prefer a stiff keeper, but any of these are stiff enough to be used without significant problems.