The heart of the plotter is the printer head assembly, consisting of a horizontal bar and, attached to it, the head assembly holding the pen in use. The pen can be positioned horizontally by moving the pen assembly along the bar. Vertical positioning is achieved by either moving the bar (stationary page plotter) or the paper (rolling page plotter). Combinations of horizontal and vertical movement are used to draw arbitrary lines and curves in a single action, in contrast to printers which usually scan horizontally across the page.
Colour plots can be made by using more than one pen. Older plotters required a separate pen for each colour and the pens had to be changed by hand. Modern colour plotters usually use only four pens (cyan, magenta, yellow, and black, see CMYK) and need no human intervention to change them.
A graphics printer that draws images with ink pens is called Plot printer. Plotters actually draw point-to-point lines directly from vector graphics files. The plotter was the first computer output device that could print graphics as well as accommodate full-size engineering and architectural drawings. Using multiple colored pens, plotters were also able to print in color long before inkjet printers became an alternative. See drum plotter, flatbed plotter, electrostatic plotter and inkjet printer.
