In older road bikes, they’re on the downtube or on the ends of your drop bars.
On mountain bikes, the shifters are mounted on the handlebar.
Internally geared hubs: Instead of derailleurs, internal hubs have the shifting mechanisms inside the hub of the rear wheel.
Internal hubs can work with a chain or a belt drive.
There are two styles of shifters popular on mountain bikes: Thumb shifters have two levers for each hand—one lever moves the chain up through the gears and one moves the chain down.
On one hand, the top lever makes the gears harder, and on the opposite hand the top shifter makes the gears easier.
Each shifter controls one cable attached to one derailleur.
On road bikes, the shifters are mounted either on the handlebar or they’re integrated with the brake levers.
The crankset, rear cassette, chain and derailleurs are known collectively as the drivetrain, pictured here: Chainrings: Bikes have one, two or three front chainrings, also known as the crankset. Cassette: Your bike’s rear cassette is the stack of cogs (gears) mounted on the right-hand side of your rear wheel, with the small cog farthest from the wheel and the large cog closest to the wheel.
Each chainring has a number of teeth on it where the chain connects.