Staedtler Technical Drawing Pens and Refills, 4 Sizes, Black Technical Drawing Pen set combines classical elements of technical drawing and writing to offer the ideal tools for sketching, graphic design and writer. Packaged in a sturdy reusable plastic case. Refill cartridges are easy to exchange through pen sleeve. Smudge-proof, resists bleeding. Black pigmented ink is water-based, fast-drying and acid-free. Set includes one barrel with four different line width cartridges: .25mm, .35mm, .50mm, .70mm. The barrel is refillable with new cartridges that include a new ink supply and a new tip. Tips won’t clog even when caps are left off for up to eight hours.