For years, shifting gears on a road bicycle called for feel: the rider moved a lever and found the position where the chain ran quietly on the chosen sprocket. In 1984, Shimano introduced its Shimano Index System, or SIS, with the Dura-Ace 7400 series. A mechanism inside the shifter gave each gear a distinct lever position.
The click at the lever is more than a sound. It marks a measured cable movement that works with the rear derailleur to guide the chain to the next sprocket. Shimano notes that accuracy depends on the whole system: shifter, cable, housing, derailleur, chain and sprockets. Indexing made shifts more repeatable and reduced the need to hunt for the right lever position while riding.
The change did not make bicycle maintenance automatic. A stretched cable or poor adjustment can still leave the chain noisy despite a clear click. For the rider, though, it was a real difference: a gear could be selected with one defined lever step rather than guesswork. The same principle remains familiar across many modern mechanical shifting systems.