Abstract: A universal, snap-on pen cartridge for attachment to the movable pen arm of a chart recorder, comprising a pen body having an ink reservoir and stylus connected therewith and adapted for engagement with a chart carried by the recorder, together with a resilient spring clip engageable with the body and adapted to clamp against an end portion of the recorder pen arm. The said end portion is substantially flat, and is held against a corresponding flat surface of the pen body. A raised portion on the body engages the end of the pen arm and constitutes a positioning device which supplements the clip action. The arrangement is such that assembly of the pen body to the arm can be quickly and easily effected, while at the same time there is provided a connection which is firm and free of undesirable looseness or slippage.