What are scantlins and treenils?

Hi
I have just acquired an old 2-foot, 2-fold carpenter's rule.
It has some really odd tables punched in it - with headings like 'sqr
scantlins to a load',' treenils' and 'ash load'.
The 'ash load' table mentions things like plow beams and car fells.
I have put a bunch of pictures of the rule up at
http://www.planimetervault.com/tree/tree.html

Can anyone please explain some of these terms for me?
Or point me at a reference where they are explained?
Thanks