Veritas 300mm Edge Rule

This anodised aluminium extrusion has a square internal angle
The outer edges are bevelled and graduated in mm with zero on the left. One edge is 9.5mm wide and the other 19mm. A high-friction strip on the inside of the wider edge helps to hold the rule in place.
Now for the good bits; the rule sits on your work and ensures all your setting out marks are parallel to the edge or the face. This straight edge rule guarantees more accurate marking out and minimises creeping errors. Dimensions can be exactly marked both on the face or edge of a board. Laying out or marking on a narrow edge with a standard ruler can be somewhat hit or miss. Especially if the rule is wider than the stock.
When setting out or marking accuracy is paramount, this tool will help you gain a higher degree of accuracy.