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There is actually no need to define head hex sizes. Our British ancestors, to wit; Mr Whitworth, had a perfectly logical system. The spanner was determined by the important part, the bolt shank size, eg, a 5/16" fastener used a 5/16" marked spanner. He had determined what he considered the optimum hex size by calculation, that became a standard, and the man on the tools selected his tool to fit the relevant shank, easy.Another weird way of defining head hexagons is wanting a spanner for a BSW bolt 5/8 or so, you cannot tell from that type what size the hex head is across flats. Got a different screw with unknown thread, so what spanner will you hunt in your garage for this case ?? So get out a vernier and take the size. Then look up a file to compare that size to any norm that happens to have same spanner ?
Unfortunately, it came undone when the head size standard changed by 1/16" to BS, British Standard, that does create some confusion, and requires a little more simple education.
The real confusion came after WWII when Captain America bullied the British into "Unifying" thread systems across the pond on the premise that if they were to fight wars together again, then they should share a common thread system across their munitions to avoid some of the mechanical disasters which reputedly happened as a result of not being able to screw each others bits together. (for want of a better expression!)
The US conceded to a few minor changes to what had been SAE and called it "Unified"
They must have been a little disappointed when the UK "metricated" in the '70's along with us Antipodeans & the Canuks!
To bmetcalf & Mike 40M's point regarding vehicles which suffered the evolution of starting with BS Standard fasteners then progressed to Unified, British aircraft which had a longer life, could have every thread system known to industry, Whitworth, British Standard, Unified, and Metric, not to mention weirdo fractional inch hex sizes which fit neither BS nor AF standard spanner sizes. Long live Messers Rolls & Royce!