Industrial Tools

A "flange" is plate shaped like a circular ring provided with holes for the studs, which attached to the end of a tube, is used to tie it with other elements.

The flange is the circle with holes inserted and welded around a pipe to connect them together. Of course in those holes appropriate bolts are placed and sometimes flange protections of rubber or other material, are inserted between the two flanges, in order to better close the joint and prevent leakage of liquid or gas.
In other words, a flange is a mechanical coupling section between two parties not directly welded, not permanent, and united through bolts or screws.
The arrangement and number of holes are called "flange jig"

Depending on the nations, flanges are united in size and jigs: in the USA they are called ANSI flanges , DIN in Germany and UNI in Italy, etc.

Each flange standards (for instance ANSI flange) has different dimensions than another flange (for example DIN flange).

In short, diameter of the holes, drill holes and number of holes never coincide from one country to another.