A friend of mine is part way through the guitar repair and construction course at Southeast Technical College in Minnesota, and we were talking about technical stuff - he'd never even heard of C-Thru rulers, without which I can't even measure... umm, stuff. They're great for parallel lines, alignment of bridges and such - I use the 2"x24" and 2"x12" whenever I need for setup, and I have every corner stocked with the little 1X12 inchers. They're like a... SECRET WEAPON! 
http://goldstartool.com/ShopCart/impl/home.php?cat=23&page=1
If you want to get totally weaponized, there are even grids big enough to lay out a whole body:
http://store.junetailor.com/store/p-54-shape-cut-pro-ruler.aspx
I have one of June's 4"X 18", and it's badass - Grandma's been hiding all the good stuff.
http://www.southeastmn.edu/academic_programs/guitarrepairbuilding/index.asp?programID=10
http://goldstartool.com/ShopCart/impl/home.php?cat=23&page=1
If you want to get totally weaponized, there are even grids big enough to lay out a whole body:
http://store.junetailor.com/store/p-54-shape-cut-pro-ruler.aspx
I have one of June's 4"X 18", and it's badass - Grandma's been hiding all the good stuff.
http://www.southeastmn.edu/academic_programs/guitarrepairbuilding/index.asp?programID=10