And on musical threads of a sort. I'm toying with making a desk mic stand and I looked up the threads and found:

5/8 x 27 UNS
3/8 x 16 BSW
1/4 x 20 BSW (also camera tripod mounting)

3/8 x 16 is the one I want and taps and dies are easy enough to find cheaply (set of 3 carbon steel taps and a die cost me £10).

I'm glad I didn't want the UNS (Unified special) thread as the only UK suppliers I found quoted a RRP of about £70 for a single tap, although one was offering one for around £25 and unlike the BSW ones, I doubt if this thread is used for much other than mic stands.