A motorised wheelhouse drives a cable, which is attached securely to two large cabins capable - in modern systems - of carrying up to 200 people. As one cabin is winched up the mountain, the other travels down.
Caroline Stuart-Taylor, managing director of the Ski Club of Great Britain, said: "In general, they're one of the safest ways to travel. But when they go wrong, it is catastrophic." That is because cable cars are used to cross wide voids, whereas the gondola-based system, in which each cabin carries a handful of people, runs up the sides of mountains and has intermediate towers. That eases the stress on the cable in a way that the cable car design does not. If the cable system fails, there is also further to fall.
At the Nevis Range Development Company, which runs one of the three enclosed chairlift systems in the UK, Miriam Austin, a spokeswoman, said: "I would be surprised if it really was a failure of the cable itself. It is more likely the clamp connecting the car to the cable failed somehow."
The cables used are high-strength steel and need replacing every 25 years or so. The design has proved itself over the years: the only difference is using computer systems to monitor the cars.
Yesterday's accident, in which 20 people died, was the worst such incident in France, said Alain Soury-Lavergne, technical director of France's Cable Car Union.
The world's worst such incident was in 1976 on a gondola lift near Cavalese, in Italy, where 42 people were killed. Itwas blamed on operator error.
In February last year, 20 people died when an American Marine jet cut a cable of a ski lift and a gondola plunged to the ground at a nearby location.