As Compig says.
Also note that early spanners, pre-WWII will have 9/16" W or 9/16" Whit (for example) while WWII and onwards will also have the rationalised BSF and Whit sizes marked on, eg: 9/16" W 5/8" BS.
The UK government brought in the standardisation to save material and to give a standard fine thread, so you can get an idea of the age of a spanner by the markings.
If you want to REALLY get confused, I have a set of SNAIL brand spanners which have Whit/BSF size at one end and A/F Imperial size the other, this particular one I have in my hand is 9/16" BS (Always BS = BSF, W = Whitworth) and 3/4" AF the other end.
Pare