The Tail Up
The Tail up is a greeting, common to our 'hello'. It also indicates that the cat is relaxed, alert and generally in a happy mood. The signal is often accompanied by a chirping noise and often is followed up by rubbing up against each other or at least the acknowledgment of the other's presence while passing through their personal space on your way to your destination.
For humans, it is a very recognisable signal that your kitty is stressfree and happy to see you and can be a handy tool in gauging just how well your kitty is coping with changes and a new situation in general - along with eating and using the litter tray.