Oh - rulers. I have the yellow/black printed ones in 6X12, 6X18 and 14 square. Then I have assorted black or blue printed ones in 6" square, triangles, circles, little suares with lip.....I don't know why I have so many. I'm sure they come in handy sometimes.
When in doubt, I use my drafting tools from days gone by.
As far as rules go....I sometimes skirt them, often break them, and generally observe the ones that keep me part of the QB.