The Mark of the Crown was a tradition on Gala for indicating the rightful heir to the throne. The Mark was very subtle and could only be detected by a specific test.
The Mark was known to the , and they tested the individual in a certain room deep within the palace for the presence of the Mark. On the blue mosaic floor of the room a silver pattern marked a certain square on the floor. Stepping on the square activated a sensor system which covered the being in a cascade of lights. If the true heir stood on the tile, they would be outlined by the light, otherwise there would be no shadow.
- The Complete Star Wars Encyclopedia