Old devices were designed in such a way that they could only be used to belay a person off of a harness. There's nothing wrong with this when one belays a leader or topropes. The problem arises when dealing with a second. Some climbers elected to belay a second directly in line, while others redirected the load through the anchor. Neither of these techniques were that effective. They both provided the leader with additional technical problems.
on the market in the U.S.
In the late 90s, climbers and guides began to use autoblocking devices to belay a second. These devices allow one to belay two ropes simultaneously directly off the anchor. If the second falls, the anchor feels the weight, not the belayer. The best part of these devices were that they automatically locked up, immediately arresting the second's fall.
Today most climbers use one of two of these autoblocking devices that have the market cornered. The Black Diamond ATC Guide or the Petzl Reverso 3 should be considered standard equipment on every climber's rack.
--Jason D. Martin