On multi-speed bicycles, gears are usually changed by a derailleur.
When a gear lever is shifted, the derailleur moves the bicycle chain from
one sprocket to another by pushing it away or toward the bicycle frame.
Multi-speed bicycles typically have a front and rear derailleur. The
front derailleur alters the available shifting range by moving the chain
across the chain rings. The rear derailleur differentiates between
the finer gears by moving the chain across the cogs. Slack in the
chain is taken up or let out by a spring-loaded arm located under the cogs.
Derailleurs were invented in 1899, but popularity only grew after the
advent of paved roads. Derailleurs developed in the 1980s began
making use of index or click shifting. These provide a click when
the shift of gears is complete, and eliminate the guesswork of previous derailleurs.