The Rotring Rapidograph uses Rotring's unique capillary cartridge. The pressure-equalization system including ink helix is part of the ink cartridge, so that every time you change the cartridge, you get a clean new helix. This means that you never need to clean the ink helix, and ensures top reliability. Ideal for tracing paper vellum drawing paper and lineboard. Line widths for drawing per ISO 128 and DIN 15 and for lettering per ISO 3098/l-DIN 6776. Drawing tube is 3mm dia. metal bushing.