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Comment by fourthark | Hacker News Reader

Infuriating that we still use the foot but it is legally defined based on the meter!

The US foot has always been defined by the meter. Originally, Thomas Jefferson set the foot to be exactly .3 meter. The science of surveying and measurement weren't great in the late 18th and early 19th c, so the definition of the meter changed; by then, the US surveys were too far along to change.

My family has been involved with surveying on-and-off for ~100 years. I hated grabbing measuring tapes as a kid, because the "foot" or the "inch" was always sone batshit crazy thing. Like: decimal foot based on a 30cm foot; the "inch" is 3cm.

No, until shortly before the convention of the meter (1875) the US used imperial prototypes provided by the UK. There was some divergence after the fire in the Palace of Westminster destroyed the originals, and the replacements were not very high quality, wrt the rapidly improving manufacturing tech of the time. I can’t remember if the US got a copy of the mètre des archives during this time, but both the US and UK defined (different) official conversions between metric and imperial / US customary. After the new BIPM prototypes were made in the 1890s, they became the standards for high precision purposes in the US and UK. The imperial / US customary conversion factors reconverged over the next few decades, except for the US survey foot.

That makes more sense, thank you.

Wait until you learn about shrink rules, used in patternmaking!

Cast metal shrinks when it cools and solidifies. To make patternmaking tolerable, shrink rules are scaled up by the shrinkage allowance so that 5" in the pattern doesn't have to be scaled by the patternmaker to some random measurement. Unfortunately, the shrink allowance varies among metals...

A steel framing square also has some completely cursed scales. I'm not a carpenter, and I won't even pretend to know why anyone needs inches graduated into 10ths or 12ths.

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