The leader must not fall.
That was once the rule of rock climbing. Still is, to some extent—falling's never a good thing, when you're suspended high above the ground. But in the early years of rock climbing, when teams of climbers were inventing new gear and scaling vertical faces that no one ever had, the stakes were higher. The ropes that climbers used, made of manila or hemp, were relatively weak and often could not withstand the force of even a short fall.
If the leader fell, the rope could snap. Or worse, it could drag the rest of the team down—and then snap. The mountaineer G.D. Abraham wrote in 1916: "The parting of a rope to which a climbing path is tied…is a frequent accompaniment of an accident. Yet this generally means the leader has fallen, and but for the breakage of the rope the rest of the party must have been dragged down."
In the early years of the sport, rock climbers eschewed most technology. They might wedge pitons (metal spikes) into the rock—but some thought even that amount of help diminished the accomplishment. But as the sport became more popular, more climbers started using more gear. By the 1910s, a team of German climbers had developed a more modern system of pitons, carabiners, and belay ropes.
It wasn't until after World War II, though, that the rules really changed. In 1934, a DuPont company scientist, Wallace Hume Carothers, had discovered a substance that was named "nylon," and by the 1940s, militaries were using the material to make ropes and training their soldiers to use those stronger, better ropes to help scale mountain ranges quickly.
After the war, serious rock climbers started experimenting with nylon ropes, too. They were certainly stronger than the old ropes—but it had some disadvantages. Nylon tended to melt, for instance, when friction heated it. In the 1950s, though, a German company created "kernmantle rope"—a layer of woven fibers protecting the stronger core of nylon ropes inside.