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What is the difference between an internal hub gear and a derailleur gear?

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An African American family of four, all wearing helmets, rides bicycles together on a tree-lined country trail, with two children in front and two adults following behind. — bicycle Family bicycling on a country road.

Derailleurs and internal hub gears are both mechanisms used to adjust gear ratios, allowing a cyclist to maintain an optimal pedaling cadence across varying terrain.1

Derailleur Gears

A derailleur system functions by physically moving the chain between different-sized sprockets on the rear wheel or chainwheels on the crank. By changing these combinations, the system alters the ratio between the crank speed and the rear wheel speed. Modern systems can offer up to 30 gear ratios and utilize spring-loaded pulleys to manage chain tension.1

Internal Hub Gears

Internal hub gears house the gear-shifting mechanism within the rear wheel hub itself. These systems are typically available with a fixed number of speeds, such as 2, 3, 4, 5, 7, or 14. While they provide a contained shifting environment, they are generally considered slightly less efficient than derailleur systems.1

In summary, while derailleurs rely on external chain movement across multiple sprockets, internal hub gears operate through an enclosed planetary gear system within the hub.