Abstract: An instrument pen system employing disposable fiber tip pens comprising relatively flat oblong cartridges having a depending nib for engaging a chart. The pens are mounted on the ends of instrument arms and the nibs are angled to enable their tips to record lines very close to each other on the chart. The flat shape of the cartridges enables the cartridges with the shorter nibs to move between the chart and the cartridges with the longer nibs. The cartridges are formed with integral lugs which enable a cartridge to be easily mounted on or removed from the instrument arm while still maintaining the low profile.