My kitchen cabinets have columns of holes drilled in the sides. Pegs can be put in the holes to support shelves at various heights. At the hardware, these pegs seem to come in two common sizes: 5 mm (simple enough), and 6 mm / ¼ inch. Of course, ¼ inch isn't quite exactly 6 mm. It's 0.35 mm bigger. Which isn't all that much. But even from the same hardware store, among the different styles of "6 mm / ¼ inch" pegs, some will go into the holes in my cabinets, and some won't. I don't mean they're tight or extra snug. I mean won't even start in the holes. And these packages are all interchangeably marked "6 mm / ¼ inch", and I've ended up just buying a package of each to bring home and try, and take back the ones that won't fit. It seems like some of the manufacturers mean "6 mm or about ¼ inch" (those fit), and others mean "¼ inch or about 6 mm" (those don't). The weirdest thing is, these are like 50 year old vintage US-made cabinets. I wouldn't have thought they'd have been made to a metric standard back then. But it really does seem that whoever drilled the holes really meant 6 mm and not ¼ inch.
i built some bookshelves about 20 years ago using those, and a tool/template to drill the holes. i think they were quarter inch. we have newer cabinets with euro style soft close hunges. whwn they turned out to be defective, the mfg went belly uo, and no others could be found to fit, even though i thought they were all the same spec. fortunately after quite awhile , the cabinet mfg actually managed to get a company in china to make replacements.
I've taken the same selection of pegs to another set of cabinets in another kitchen from around the same era, and sho 'nuff, the really-6-mm pegs go in nice as can be, and the really-¼-inch pegs do not. And I don't think there was anything exotic or euro or offshore about these old cabinets in utilitarian Indiana kitchens. One set still shows labels saying built to the ANSI A161.1 standard of 1973. It looks like the next edition came out in 1980, so I guess that kind of brackets when these were made.
We had a kid's bike requiring 25 mm diameter seat post. Tried to get replacement from seller (Sears), and they ended up wailing away, trying to get a one inch (25.4 mm) post to fit. It's sad the metric system originators did not foresee the need, to establish easily convertible units, with some common ground. Trips everyone up in industry too: there's a legacy of a maddening amount of extra work. We would receive equipment drawings almost invariably in imperial, convert them all to metric for support steel working drawings, and hand them to a fabricator who generated shop drawings in imperial once again.
Mendel, I worked for ITT in the 1980s. ITT sold telephone equipment world-wide into both Metric and Imperial markets. The systems had measurement units as the basis for their drawings in all markets to hide that all of the measurements were actually based on inches so that they could be sold into Metric markets without complaints. JeffD
Apparently, France had approached America and Britain about working on it together, but that ended up not happening. As a child of the French Revolution and Enlightenment, the metric system system wasn't going to reference an arbitrary system based on some king's foot or mayor's favorite goblet anyway. They wanted something with rational rules, that had the measures of length and weight connected to the other, and that the standard was tied to something of the natural world. A decimal system made doing math for science and engineering easier. The original meter was one ten millionth of an Earth quadrant. The cubic decimeter labeled a liter for volume. A milliliter, or cubic centimeter, of water's weight defines the gram. The standard meter has changed over time; it is now how far light travels in a vacuum in an iota fraction of a second. Need to waste time in Wikipedia. https://en.wikipedia.org/wiki/Metric_system https://en.wikipedia.org/wiki/Units_of_measurement_in_France https://en.wikipedia.org/wiki/International_yard_and_pound
Yeah I recall one overland conveyor I worked on: the belt covers the client wanted were 4"-0" spacing, and with the job being metric, that converted to 1219.4 mm (funny how that's memorable...). So EVERYTHING on the overland tables, trusses, and head-end structure was on a cadence of 1219.0 mm, to accomodate these effing belt cover bolt spaces. And then the client switched to a different belt cover, no such problem. But the horses had bolted by then. There's countless other examples, I better not get started lol.
And then here we are in our Japanese cars where the oil pressure switches use a British standard pipe thread.
