Does anyone know how G.P.I. thread sizes are arrived at?

Does anyone know how G.P.I. thread sizes are arrived at? They seem to be used especially in the bottling closure market with such results
as: 24/410, 36/400, 28/415
It seems that the digits preceding the slash are the mm diameter of the thread to its outer-most point but I can't seem to work out what the post slash information means.
Thanks for any help
Rod
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try machinery's handbook
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