As has been so well illustrated in previous replies to your question, it was Dunhill and their usage of the phrase "group size" to denote the size of a specific pipe it was as can be seen from Dunhill's own shape chart to be a somewhat arbitray thing that was really established on the shop floor prior to stamping to establish the selling price for each pipe. Over the years there was some consistancy in this method and many folks use it as a general reference for sizing other pipes. Just to add another wrinkle in all this, Charatan used a similar numbering system and reffered to heir pipes bt grp sizes as well only Charatan's sizing was approximatly 1 1/2 times LARGER !! A Charatan grp 4 is slightly larger than a Dunhill grp5 and depending on the shape can be as big as a Dunnie grp6!! Ah the PRECISE SCIENCE of pipe smoking !! :twisted: :twisted: