A convertible writing instrument can be put into either a compact capped configuration, 3 to 4 inches in length, or an extended writing configuration, 5 to 6 inches in length. A relatively elongated cap, which covers most of the relatively shortened pen member in the compact configuration, can be re-attached to the back of the pen, so as to lengthen the instrument in preparation for writing. Multiple interchangeable caps and rear modules can be coupled to the pen member to provide a variety of appearances and functions.