Tilton RaccoonMan, I thought US lumber sizes were fucking weird, literally EVERY TRADE is fucking weird here. I need NM wire clamps for our main electric service box. The punch-out hole measures 7/8" in diameter (22.5mm). The trade calls this a 1/2" punch-out, but you don't buy a 1/2" clamp, you buy a 3/8" clamp. So yes, you need a 3/8" clamp for a 1/2" hole that actually measures 7/8". Thanks, I hate it.
Charlie Stross@tilton @a_cubed And inches are 25.4mm give or take a few microns! I mean, who ordered THAT? It's almost as crazy as the pre-revolutionary French Foot, which was the distance from the tip of the king's nose to the end of his right middle fingertip (arm extended stiffly from the shoulder) because why the fuck not (and also, it varied from king to king).
Jeff@cstross @tilton @a_cubed NIST ordered it, and there's no give or take. A US inch is exactly 25.4mm to as much precision as your equipment allows. That is the official definition, and why I like to call US customary units The Worst Version of the Metric System. NIST redefined everything in terms of metric units half a century ago.
@overeducatedredneck @tilton @a_cubed Oh good grief. That's as bad as the British Metric Pint (568ml precisely, legal/statutory measure for serving beer in pubs, used for NOTHING else.)
Nick Taylor@cstross The UK has also been on the metric inch since like the 1930s. Saved our bacon in WWII because American-machined parts *weren't* a few thou per inch out.
Of course, in practice both the US and the UK had been on the metric inch for a while before then because the (I think Swedish) chap making the gauge blocks got fed up with having to maintain two product lines and split the difference between the two inches.
Andrew Dalke@tienelle The Swede Carl Edvard Johansson's gauge blocks, to be specific.