Well, it is kinda confusing. Round tubing is measured on the OD, pipe is measured on the ID (kinda). 100 yrs ago when pipe was manufactured, an 1-1/2 pipe would have a ID of 1-1/2 and an OD 1.90 and pipe is alot of the time threaded on the end. With better steel making process, the wall thickness could be reduced and still carry the same pressure. Now that pipe has a ID of about 1 5/8. It still needs the same 1.90 OD for the pipe to be threaded. So basically pipe has weird OD sizes up until 14" dia and bigger where pipe is measured on the OD and 3, 3-1/2, 4, 4-1/2 have 'normal' OD sizes. Like Austin Powers once said. 'Oh dear, I think I've gone cross eyed'. I guess you just learn when you've used it enough.