X Mark

The "X" Mark is something the first 6 Straw Hats (Luffy through Chopper), as well as Nefertari Vivi and Karoo, have on their forearms.

During their Arabasta adventure, they had to face a very tricky opponent, Mr 2 Bon Kurei. He, with his Devil Fruit Power, can turn into anyone of them, so Zoro came up with a very good idea, which excited Luffy. They made a black X mark on their left arms, as a symbol of their belonging to the Straw Hats.

The X Mark came into play several times.


 * When Vivi is heading toward Alubarna, the capital city of Arabasta Kingdom, she met Usopp, but she wasn't sure if he was him or not. As she asked him to show their recognizing symbol, he showed a bandage, but without uncovering the X mark. Vivi understood that he was Bon Clay disguised as Usopp and ran away.
 * When the Straw Hats were leaving Arabasta, they could not say good bye to Vivi in a proper way, to avoid the princess being recognized by the marines as their crewmate. So, they stand with their backs facing Vivi, raising their left hands and showing their X. Vivi replied in the same way too, as a symbol of their unbreakable friendship tie.

The X Mark hasn't been seen since the Arabasta arc.