do you know why "mm" (which to me means "millimeters") is used with $ values? I mean, a house can be $ 250K, a mansion might be $ 1.1M, and our national debt might be $ 34.4T. OK, I understand those.
How does 'mm' equate to 'M'? And if it does equate, why not use 'M' as a simpler way to designate a quantity of "millions" ?
Western finance/accounting industries adopted ‘M’ from the Roman numeral for 1,000 to mean “thousand”. MM (or mm) meaning “thousand thousand”, or a million.
Separately, when the French invented the metric system they used the Greek prefixes for multiples (kilo, derived from the Greek for “thousand”, being the best known). Which is why ‘k’ denotes thousands in most other industries.
Conversely, the metric system used Roman prefixes for submultiples, which is where centi- (same root as Centurion) and milli- come from.
As for SI, I have never liked the 'non-thousands' submultiples e.g. centimeters. I think it's confusing. You have km, m, and mm. factors of 1,000. Seems reasonable. Just like we have F, mF, uF, nF, pF for capacitors.
I think the various units are there because people like 'comfortable' numbers; e.g. people seem to prefer 8.9 cm to 89 mm ('8' is a number between 1 and 9, the most preferred numbers -- because we can count them on our fingers?).
How does 'mm' equate to 'M'? And if it does equate, why not use 'M' as a simpler way to designate a quantity of "millions" ?