This pack of two classic size rollerball refills have been designed by Montblanc to fit a variety of their rollerball pens. The screw-in refills are designed to fit securely in your pen when writing and when on the go. The mystery black refills feature a liquid ink inside that works with the fine rolling ball nib to create a smooth and thin line on the page. The fast-drying ink helps to prevent smudging on your page to keep it looking neat and tidy.