I am looking for some clarification on Class A surface models. I understand that Class A surface models need at least up to G3 in surface continuity. I also understand what this means. However I am trying to find out what G4 G5 G6 mean? Does anyone know of some references that I could find out the different meanings of the G codes. I am researching these for a paper and I have been hitting a wall. Any help would be greatly appreciated. Thanks in advance.