Getting rather confused about compression fittings and thread sizings.

Take a 15mm straight coupler, e.g.

http://www.screwfix.com/app/sfd/cat/...88231&id=14921

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:

http://www.screwfix.com/app/sfd/cat/...12341&ts=05907

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?


Help!

Graham