The pen assembly holds and moves the pen over the paper. It depresses the pen into the paper wherever the image is desired. A small cylindrical holder, attached to the printer head assembly, houses the pen. The printer head consists of a horizontal bar along which the pen housing may be moved. This positions the pen at any 'X' coordinate on the paper. The horizontal bar can move back and forth, enabling any 'Y' coordinate to be selected. This enables the printer head to select any point (or series of points) on the stationary page. Other plotters have a single bar which moves the pen back and forth as the paper is passed through the plotter. The friction feed of the paper controls the 'Y' coordinate in this type of plotter. In some, higher priced plotters, a number of different colored pens rest in a cylindrical drum beside the printer head. When a different color is desired, the plotter can auto-select the appropriate pen.
