It began a key ring, and was developed into a writing pen. Printed at full size, any ink pen core should fit nicely through the tube. It says eternal on one side, as it was intended to be given to customers. A side note, the support material is DIFFICULT TO REMOVE completely, especially the tube where the pen goes. As such, a 9/64 drill bit or smaller should do the trick. There is even a hole on back top for a key ring. Also an ink pen cap cut shorter will snugly fit tip.