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Reverse gear is usually marked 'R' on the selector. The forward-drive clutch is disengaged and the first ring gear freewheels.

The reverse clutch is engaged and the engine drives the sun gear.

The second planet carrier is braked and the sun gear causes the planet gears to drive the second ring gear in the opposite direction to give reverse drive.

Explained in

    How automatic gearboxes work

    Most modern automatic gearboxes have a set of gears called a planetary or epicyclic gear train. Read more