The D1 67mm pen refill is a standard pattern used in most multi-function pens and many compact or expanding pens. Made of metal with a hollow end for mounting on the pen, it measures 67mm in length and approx 2.3mm in diameter. We have D1 refills from Lamy, Schneider, Parker, Zebra, Tombow and Faber-Castell and thanks to the huge variety in pen types that use them there is a good choice of ink colours including black, blue, red and green. Due to the small ink capacity medium point refills are no longer used - all the refills here are fine point, though the Parker refills are probably the broadest of these. In addition to ballpoint refills there are highlighter refills (in actuality a bright ink pen, as normal highlighter tips can't be accommodated in this format), and stylus tips. If you have a multi-function pen you can normally swap any of the functions for alternative refills - eg. change that little-used red ballpoint to a PDA stylus tip. One refill worthy of note is the Tombow BR-VMP refill which is pressurised - meaning it can write upside down, in cold conditions or on wet paper, just like a Space Pen.
