We use this because the widest point for a shirt is not the same as your trouser’s waist measurement. For example if you are a very lean but big guy, you may have a jeans size of 34 around the waist, but measure 32 inches around the narrowest part of your waist. On the flip side, if you have a bit of a belly, the widest part of your waist may be larger than your trouser’s measurement.
For those of you who are lucky enough to have an athletic physique. You might not quite have a six pack but are not far off!
This is more for the average guy who is in pretty good shape, but not going to prompt squeals of delight from any nearby ladies when taking off his shirt! We are thinking of Colin Firth in Mama Mia! rather than Pride and Prejudice.