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.