This may not be logical, but long usage has made it permissible or even necessary.
Voluntary enlistments in the French army are permissible, within certain limits, at the age of eighteen, and the engages serve for at least three years.
He would sometimes regret that it was no longer permissible to leave it in the old Roman fashion.
As a rule it is not permissible to erect a building wider than the road, measured from building line to building line.
All such variations are permissible provided that the quantity of silica and alumina is sufficient to saturate the whole of the lime and to leave none of it in a "free" condition, likely to cause the cement to expand after setting.