There are two basic types of pvc pipe flanges standard and Van Stone pipe flanges. On a standard flange it is one piece, that is once you glue up the pipe you cannot rotate the flange to match the bolt pattern. On a Van Stone flange, the center piece can rotate on an oring and is separtate from the pipe flange. This allows you to rotate the pipe flange after gluing to mate up with the butterfly valve, diaphram valve or whatever other flanged fitting you are mating to.