http://en.wikipedia.org/wiki/International_System_of_Units
The SI rule for pluralising units is that symbols of units are not pluralised[2], for example "25 kg" (not "25 kgs").
Reference 2 is
Bureau International des Poids et Mesures (2006). "The International System of Units (SI)". 8th ed.. Retrieved on 2006-07-14.
So this is quite specific and so I definitely think they should GO for SI units.
Of course, we're not talking about symbols of units (which, incidentally, are not pluralized in the library), but full names of units. The original rationale was that quantity<SI::length> q = 2*meters; reads a bit better than quantity<SI::length> q = 2*meter; If anything, I would argue that the singular cases are the edge cases, since you would only use them for the case of unit valued quantities : quantity<SI::length> q = 1*meter; and, by my reading, even this would be better plural : quantity<SI::length> q = 1.0*meters; Naturally, I'm eliding the multiplication in reading this expression... Anyway, as with many of these points, I'm not dogmatic. I see the merits of simplicity - how about I remove the singular cases? BTW, Paul, I hope you'll have a chance to give the library a formal review. We'd appreciate your input. Matthias