When a cat moves its tail quickly from side to side, it is feeling afraid, aggressive or both. This behavior indicates an unfriendly mood, and a cat that is whipping its tail should be avoided.
When a cat's tail moves slowly back and forth, it shows that the cat is focused on an object, such as a toy. A cat often pounces after giving this signal. If the cat's tail is upright and curved like a question mark, it is feeling playful. When the tail is straight up, the cat is confident and happy. If the straightened tail is low, the cat is usually in an aggressive mood. However, Persians and other breeds often hold this position when they are calm, as well. If the tail is tucked under the body, the cat is afraid or submissive. An annoyed or frightened cat sometimes puffs up its tail to appear larger and more imposing.