On Thu, Jul 07, 2016 at 09:59:11AM +1000, Kenneth Barns wrote:
> Lifelong metric user here, and I had never _heard_ of ares and
> steres before this discussion.

Yes, neither are nor stere are metric units any more; not even
"accepted for use with SI".  Hectare is accepted for use with SI,
but is not an SI unit.

> Perhaps that is because they never made the jump to international
> use when the metric system was formalised in the late 1800's in the
> way that tonnes, hectares, and litres have.

They were official metric units from the very beginning of the system
up until 1960, though seldom used.  "Stere" and "kilostere" are still
used in water flow management but are no longer accepted with SI.

If anything, the odd one out is "tonne", which was only retrofitted to
the metric system later from customary usage.  The "mega-" prefix was
introduced late into the metric system, and the "bar" (1000 graves)
had been eliminated very early.  So the largest official unit of mass
during that interval was the myriagram (10 kg), which was insufficient
for many purposes.  The gap was filled by colloquial use of
alternative names for 10^3 kg, including a few variants on "tonne".

The official name for the unit since 1960 is "megagram", which is in
use by almost nobody.

- Tim