I feel like I should know this, but this is the first time I've seen it outside of Shelley's books. What exactly does the 10/6, 13/6, 16/6, etc signify? I know the diamonds are at foot markers, in this case every 3 feet. But I can't make sense of the 10/6 etc. I get that it's on 18" centers but the numbering doesn't make sense to me. Why does 1/6 fall on the first increment of 18, but 4/6 falls on the third? Why is it in 6ths?