I was getting quite confused too. So I went to my only square ruler which I use a lot, 6 1/2". It has 1/4 inch allowed on all four sides. So that is what I am used to...no wonder I couldn't understand the 1/2 inch on one side and none on the opposite side...My ruler is by Marti Michell. Have had it a long time.
There, I feel better now! :-)