On Wed, 25 Jan 2006 08:00:08 -0800, wrote:

I have an old plumbing elbow, female both ends, it looks like half inch
bsp to me but is there a way to double check please. Just for info it
will thread on to a 15mm compression joint thread.

Then it probbaly is. How accurately do you need to know?
(It could be a taper rather than parallel thread)