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As you will see, the second one has curved bits on its legs. When you sit in the chair, it will tilt backwards and forwards, yet remain in the same spot.
Gary - did you check your dictionary before asking?
A rocking chair has curved blades on the bottom instead of tradtional "legs." It is designed to that the person sitting in it can rock gently backward and forward while sitting in it. It is quite soothing, especially for a mother holding a baby and trying to soothe it, or for sitting outside on the front porch on a soft summer evening, watching the world go by.