Demography

The fifth entry is le seuil de renouvellement des générations, and the noun in the middle of it is where the two languages part. English calls the same quantity the replacement level, and the picture behind that is substitution: one thing put in the place of another. Le renouvellement says something else, that a stock is made new again, and the verb behind it does three jobs that English hands to three separate verbs. It renews what would otherwise lapse: renouveler un passeport, renouveler un bail, renouveler un abonnement. It repeats an act: renouveler une demande, renouveler ses excuses. And it restocks a set whose members leave one at a time: renouveler l’équipe, renouveler le tiers du conseil, renouveler l’air d’une pièce. The third job is the one this entry needs, and French keeps a separate verb, remplacer, for the plain swap — which is deliberately not the one used here, because nobody stands in for anybody. « To replace » would be almost right and exactly wrong: the threshold counts what a generation must produce for the whole stock to be made new, not for a child to take a parent’s seat.