If you need an ultimate definition of the HP-GL/2 language join the HP Printing and Imaging Solutions Provider Program.

You can find some brief description of the HP-GL/2 and HP-GL formats here:

• http://www.wotsit.org/
• http://www.sxlist.com/techref/language/hpgl.htm
• http://lprng.sourceforge.net/DISTRIB/RESOURCES/HPGL_short_summary.html
• Wikipedia (English)
• Wikipedia (German)