SI units are assumed to be simple items like V (volt) where 'mV' would cause convert to insert: 'm' before the base symbol 'V' to make 'mV', if abbr=on 'milli' before the base name 'volt' to make 'millivolt', if abbr=off A unit like "square meter" would not work because it needs an SI prefix inserted before "meter" rather than at the beginning of the name.