Take a 15mm straight coupler, e.g.
The internal diameter if the fitting is obviously 15mm, to take naturally 15mm pipe. But what is the thread size on either end?
Now take something like:
This is described as Female Iron Coupler Compression Fittings 15mm×½"
The 15mm must relate to the internal diameter and I guess the thread size is the same on one end as the stright coupler. But what does the 1/2" relate to? The internal diameter on the other end or the thread size?