Tail position and movement offers insight into your cat's psyche. Basically a cat's tail can be up, down, or sideways; it can be curved or straight, and it can be still or moving. Here is how to interpret the various positions and movements of the tail:. Tail tucked - fearful, defensive
. Tail held at half-mask and moving slowly from side to side - indicates mild interest
. Tail vertical or straight up - indicates anticipation and/or greeting
. Tail vertical but curved to one side - indicates expectation/monitoring
. Tail held completely to one side in a female - indicates sexual receptivity
. Tail held low with tip twitching - indicates a stalking predatory stance
. Tail frantically switching in wide arcs - indicates heightened affect/aggression
. Tail puffed up (piloerect) - indicates fear and aggression
