The stem of the upper case 'i' in FF Meta Hairline is 10 em (is that the correct usage?) wide on a 1000 upm grid. I do not imagine that you would want to go much below that.

I'm working on an ultralight, and the stem of the capital I is 18 thick. I'm not going to make it any thinner ;-)

Of course this depends (at minimum) on how big the resulting font will be set.

Luc (de)Groot has done one with 1 unit. I think it is visible at 1 Meter, or was it 10?

What Craig said.

BTW, at 1 unit, can you even make normal curves?!

hhp

I think there were curves, but am not exactly sure. I designed my Agilita Hairline with 4 units which was extremely difficult, especially in the Italics. For optical compensation i went down to 3 units. And that was the end of affairs, i found. The next weight UltraThin is four times as thick (16) :-)