In modern-day cycling, multi-stage events are predominantly decided during the mountain stages.
Mountain stages are where the general classification (GC) contenders can no longer rely solely on the protection and support of their teammates, as they often do during flat or rolling stages. In the mountains, riders are stripped down to their raw abilities, with teamwork giving way to individual talent and grit. As the gradients steepen, and the air becomes thinner, it's up to the riders themselves to prove their mettle, making or breaking their chances for overall victory.