You can increase or decrease the width of the default pen in SeaTurtle using the
PENSIZE command. This is effectively the width of the lines you draw. The command takes an integer expression indicating a pen size. The default pen size in a new SeaTurtle program is 1. Here is an example of changing the pen's size to 5, drawing a line, changing the pen size to 10 and then drawing another line:
PENSIZE 5 FORWARD 30 PENSIZE 10 FORWARD 30
You can use variable pen size to draw more complex pictures. For an example of this, see the exampe program "road."