Pipe sizes were defined in the 'old days' when everything was threaded
connections and you needed to be able to flow a certain amount of gas or
liquid.

Pipe has a defined OD so all the different pipe fittings can be be
attached. The ID is sized so that "normal" wall thickness (designated
'Schedule 40')will give an ID that is equal to or larger than the
nominal dimension. After that, there are Schedule 5, 10, 20, 40, 80, and
160 to handle different pressures which give you all the different wall
thicknesses. Confused yet?

Michael Koblic wrote:
I was looking up dimensions of steel pipe. I found a few tables with
figures. Here is what puzzles me:
Take a 3/4 pipe: ID=20 mm. OD=26.7 mm. This is the same for about five
different pipes listed. Yet each pipe *had different wall thickness*. How is
this possible? Another Tardis principle?