From the Super Mario Wiki
Retrieved from "http://www.mariowiki.com/index.php?title=Card_Key&oldid=2047966"
The Card Key is an item found in Paper Mario: The Thousand-Year Door and Super Paper Mario. In Paper Mario: The Thousand-Year Door, it must be used to access certain areas in the X-Naut Fortress on the moon. The Grodus room to the right of sublevel 2 has a green Card Key on the computer table. The electrical-floor room to the left of sublevel 2 guards a purple Card Key. And the electrical-floor room to the left of sublevel 3 guards a yellow Card Key. They are necessary to open the door to the factory room to the right of sublevel 3, therefore allowing Mario to fight the leveled-up version of Lord Crump. In Super Paper Mario, Card Keys are blue with purple triangles and a purple line, and the top is white. Mario, Peach and Bowser must use it to get into the processing center in the Floro Caverns.