I was at a friend's shop the other day and he was showing me two sections of pipe, about2 feet in diameter. They were welded at 90 degrees with a flange on the downward section. I was asking how do you verify that the top pipe and the bottom of the flange are parallel? He started to explain and got as far as "standard two hole...", and got interrupted. I never remembered to ask what that meant....So..what was he going to say??