The spelling "tonne" pre-dates the introduction of the SI in 1960; it has been used with this meaning in France since 1842, when there were no metric prefixes for multiples of 106 and above, and is now used as the standard spelling for the metric mass measurement in most English-speaking countries. In the United States, the unit was originally referred to using the French words "millier" or "tonneau", but these terms are now obsolete. The Imperial and US customary units comparable to the tonne are both spelled "ton" in English, though they differ in mass.