Kevin dates the rule back over a 100 years, so we’re back in the era where many
’standards’ existed, and screws were still made individually in workshops.
Johns theory makes sense to me.  If you were hand making a nut and bolt from raw
steel (or iron) you might appreciate a bit of help.


As regards ’spanner’ - why, what else would you call the thing that spans across
the flats of a nut?    Those of us who started the industrial revolution would
look at a nut with a practiced eye and say to t’ lad “Ayup, gormless, pass us
summat that’ll span abaht seven eighths of an inch - and be quick abaht it or
they’ll get my fist rounf thi lugs”

And the gormless lad would take out his rule, measure the span of the spanners,
and select the right one.


It’s early - a little dialect word picture to start the day.  


from ‘every boys guide to becoming a Yorkshire engineer’ - 1893 edition. 

Richard Wilson
Yorkshumbrian Galoot.