Wonderful, ISO-128 is exactly it and the wiki explains it perfectly. This seems like a fantastic sizing for pen sets.
The 0.10, 0.13, 0.18, 0.25, 0.35, 0.50, 0.70, 1.0, 1.4, and 2.0 scale with √2. I couldn't tell if other sets were just rounding or dropping the hundredths place. The rotring and staedtler both offer sets in those increments. I suppose this is just common knowledge to those who need it.
The 0.10, 0.13, 0.18, 0.25, 0.35, 0.50, 0.70, 1.0, 1.4, and 2.0 scale with √2. I couldn't tell if other sets were just rounding or dropping the hundredths place. The rotring and staedtler both offer sets in those increments. I suppose this is just common knowledge to those who need it.