A Reverso is a belay device

Belay device

Belay devices are mechanical pieces of climbing equipment used to control a rope during belaying. They are designed to improve belay safety for the climber by allowing the belayer to manage their duties with minimal physical effort. With the right belay device, a small, weak climber can easily...

 developed and patented by Petzl

Petzl

Petzl is a world leading manufacturer of climbing gear, caving gear, work-at-height equipment, and headlamps based in Crolles , France. The company was created by the cave explorer Fernand Petzl in the mid-1970s...

, used for example in rock-climbing and other activities which involves rope-work. Another version of this device is the Reversino, intended for use with thinner ropes.

A Reverso can be used to belay the leader, one or two seconds, or a descender during rappelling. It can also be used in self-braking mode, to belay one or two seconds, if attached directly to the belay.

A Reverso is similar in design and use to an ATC when belaying a lead climber from below. A Rope passes through the reverso, through a carabiner attached to both the reverso and the belayer's harness, and back through the same hole in the reverso. This creates the friction required to properly belay a climber.

When belaying a seconder from the top of a climb, the reverso is attached by a carabiner

Carabiner

A carabiner or karabiner is a metal loop with a sprung or screwed gate that is used to quickly and reversibly connect components in safety-critical systems. The word comes from "Karabinerhaken", meaning "hook for a carbine" in German.-Use:...

 through the "extra loop" that an ATC does not have, to the anchors in the place of an italian hitch knot. Using this method, the device is self locking, and cannot be released without extra gear.