Love ckcowls answer. In a perfect world you could borrow a ruler first and complete a small project, then decide to keep it or not. Log cabin rulers have measurements to cut the logs down to specific lengths before making the block. If you have a tendency to stretch your fabric as you sew, and are always having to trim your blocks down, this ruler might be a good idea. I would make a couple blocks without it. If you like the way they are coming out, then you probably don't need it.