"M"
before the abbreviation of a unit means 1,000, but on its own
it is far more commonly used to mean 1,000,000.
"m" never means
1,000 - it means 1/1,000 when used with the abbreviation of a unit,
but on its own it usually means 1,000,000 too.
I beg to differ,
Thomas. It may be an Americanism (I would have to
find a source for that), but "M" is generally understood to refer to
thousands in currency. It comes directly from the Latin "Mille".