Tombow's Mono Knock eraser has a pen-style body that is easy to use and store. It features a convenient clip for on-the-go corrections, and a rubber grip for comfortable use. Round; 3. 8mm diameter. Available in Pink, Green and Blue. Tombow is passionate about offering the finest and most innovative executive pens, office supplies, and arts and crafts products on the market today. Whatever your project, from preparing documents to scrapbooking to fine art projects, you can count on Tombow for unmatched quality and design.