Measuring tape. it has to be accurate, a certain precision will defeat the purpose. They are accurate in the first place; I checked, Starrett is my standard and they match exactly. I am careful with the measuring tools I have in my shop; my Starrett and fastcap are for building fences and lumber yard rides. They are terribly yellow, I would never lose them in a pile of shavings. I will buy 4 and I will bet for life!