Re: Why mm?
"Because when building something, if your tolerances are a millimetre or so out, you can usually fill the gap adequately with some sort of filler, "
Not really. We went through this 50 years ago in Australia.
◊ You don't need any fractions – no vulgar fractions and no decimal fractions.
◊ You don't need decimal points.
◊ You can easily develop a mindset for assessing distances and lengths using millimetres; it quickly becomes second nature, and big numbers have never been a problem.
◊ You can be as accurate and precise as necessary for the job you are doing. For example in the building trade you rarely require accuracy greater than 'to the nearest millimetre'.
◊ Aiming at 'millimetre accuracy' automatically produces a better job than aiming at 'centimetre accuracy'.
◊ If you use milli as the preferred prefix, there are a smaller number of prefixes.
◊ The whole idea of avoiding the centimetre, decimetre, decametre, and hectometre in favour of the millimetre is to eliminate clutter and achieve simplicity.
◊ By preferring the millimetre, people have to work with only three length units: millimetres, metres, and kilometres and anyone who uses those three length units simply doesn't bother with any of the others. This matches our already established practices of having only three units for mass: grams, kilograms, and tonnes; and only three units for capacity: millilitres, litres, and cubic metres.
◊ Look at the way people so readily adopted the use of grams and millilitres without the need for centigrams and centilitres.
◊ It takes a lot less time to train people to use SI using millimetres so people learn it more thoroughly.
◊ For most people there will only be one 'conversion factor' – 1000 – in the whole system if they choose millimetres instead of centimetres.
The building industry changed from imperial to metric in two years or less. The textile industry chose to use centimetres and still haven't completed the changeover. Today there are nearly a hundred occupations using millimetres and 12 using centimetres.