Answer
If you look on the HSW page on motors you will see that the field magnet and the rotor magnet attract and repel one another at different points in the cycle. The commutator is in charge of flipping the rotor's magnetic field. The direction or rotation therefore depends on which way the current is flowing through the rotor. If the motor is driven by a battery and you flip the polarity of the battery, the direction will reverse.

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