This set of 2 FriXion Ball Version 2 metal refills for FriXion Ball Knock Zone compatible with FriXion Ball Knock Zone which uses a newly developed "Premium FriXion Ink" that allows users to write darker and longer. Premium FriXion Ink is enhanced by 30% in black and 15% in red and blue than traditional FriXion ink, so you can write dark and high visibility.