Knitting Needle Transalations
(or nations separated by a common language)

I am pretty clued up on translating between imperial and metric units of measure, both in fabric sizes and old UK knitting needle designations. My formative years were spent while the UK was changing from one system to the other; sometimes I change units mid-sentence.
Something I was made aware of only recently, though, is that the old UK sized needles in my possession are not necessarily exactly rigorously the same size. I guess this is pretty obvious if you think about it, given that knitting does not rely on engineering levels of tolerance, but it had never occurred to me before.
I would mostly say this doesn't matter - especially if you check your tension with a swatch before you start - but I often use a mixture of needle pairs for one project. It was actually noticing the difference in tension that made me suspicious and check it out. [I have been known to unknowingly change needle sizes during a project - but let's not dwell on that].
The examples that brought it to my attention were "number 9s", and I was able to check it using my needle gauge, which has a mixture of old and new (millimetre) sizes.
The old UK sizes (for example, 9 through 13) did not translate sensibly to mm. You end up with 3.75mm for 9s, and 3.25 for 10s and so on; if you buy needles in France, you can see they have, for example, 3mm (11s), 4mm (8s), and only one size (3.5mm) in between, as you might expect. Some of my needles labelled "9" erred more towards 3.5mm than 3.75mm, and amazingly it did show up in my knitting.