pen-down (short version pd) is used to trace the movements of turtles. The color of the pen is the same as the color of the turtle. While pen-down starts tracing, pen-up (short version to pu) stops tracing a turtle’s movement. Think of pen-up and pen-down like a pen on the belly of a turtle; pen-down lowers the pen to the ground, leaving a mark of its path, while pen-up pulls the pen up, no longer marking the ground. Pen-down and pen-up are primitives used only with turtles. For example, if we wanted our turtles to draw a square, we would write the following code:

ask turtles [
    pen-down
    repeat 4 [
        right 90
        forward 5
    ]
    pen-up
]

In the model below, an airplane is flying between two mythical destinations: Atlantis and Valhalla. As it is flying, it leaves a trail behind it if the draw-path? switch is on. Every time a plane reaches a destination and turns around, it also changes its color, which leads to a new trail color. If the draw-path? switch is off, our plane picks up its pen and stops drawing. We also use the clear-drawing primitive to clear all the previously drawn paths when the draw-path? switch is off. Notice that clear-drawing is an observer-only primitive, so we use it outside the ask turtles statement.