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The different grades of the Risser sign give a measure of bone maturity and thus of the skeletal growth to be expected. In Europe, a scale ranging from Risser 0 to Risser 5 defines the grades from beginning ossification to complete fusion of the apophysis to the iliac bone. In the USA, 4 grades are defined.
The three scales overlap and that's where the Rockwell measuring system can be quite misleading. The A scale overlaps the B scale and the B overlaps the C. So a high RA (read: Rockwell A value) can mean the same hardness as a low RB and a high RB can mean the same as a low RC. Some people who don't like this tend to prefer the Brinell Hardness scale which is significantly more simple to use.