Stewart RussellIf you value your time (and partly, your sanity) don't start reading the definitions file that comes with the gnu units(1) command. Therein you will learn more than you might ever want to know about the Gas Mark system, sugar solution concentrations and how many different lengths have been called a "point" in printing.
You can find where the file lives using this command:
units --unitsfile
Beware! Here be dragons!