13/32" - from old tool catalogs from the 1930s (= 10,31875 mm - but that is too large for today's tolerances: min 10.04 and max 10.19mm) but 0.3937" is too small and less than 10mm, sorry.
Until the 10mm nut is closer than about 15mm to something beside it. Then that widget wont work. I have one in my toolbox in the bus. I dont think I have ever reaĺly used it- I was given it a long time back. If you get it off centre it sweeps out an even wider circle. The trick is to have at least four 10mm sockets stored in different toolboxes. Then only two of them go missing at a time.
Surprised that nobody's mentioned 13mm spanners, that always vanish off the face of the earth, only to magically reappear when you've spent half an hour digging your 1/2" one out of an old toolbox.
Theres a guy at work who says he used a 13mm spanner once assembling VWs in South Africa. I DONT BELIEVE HIM !!
And when I was doing up gas connections the other day, my 15mm stubby spanner announced its disappearance by not being there.. sad.. I bought that spanner at Santa Pod while we were en route to Cambridge to pick up the vibes of the University for younger daughter ... she got a first class masters in biology at Bristol instead... maybe Santa Pod was a bad vibe..
Went o the Santa Pod ‘Night of Fire’ in my old van to watch the jet beetle Outrage. I lost my hearing…