Always have something to write with, with a pen attached to your keys! This model is designed to perfectly fit a common Bic Round Stic pen, and has a short, strong chain so it sits comfortably in your pocket.
I suggest cutting your pen shorter for a better fit in your pocket, as I have in the photo. Just pry off the business end of the pen, carefully cut the ink tube shorter with a pair of scissors, cut a similar length off the open end of the outer tube, and reassemble. Also, when you get it, there may be some excess powder stuck inside - a toothpick works wonders.