On Sat, 26 Jun 2010 03:40:58 GMT, (Doug Miller)
wrote:

Think about it: if OD were "whatever the manufacturer feels like" you wouldn't
be able to use Manufacturer A's fittings with Manufacturer B's pipes. It's
precisely to ensure such interchangeability of pipes and fittings across
brands that the sizes are specified by OD, not by ID.

Would that not necessitate that ID also needs to be specified to the
fitting of one manufacturer will cover and fit the OD of another
brand? Sounds like it works both ways.