Athel Cornish-Bowden in Marseille asks about the final -s in some French place names, and French versions of non-French place names (e.g. Douvres, Londres, Cornouailles).
The final -s in these names is often the final remnant of the Old French masculine nominative singular case, which in turn is the left over of Latin final -us in masculine names. Old French retained two of the Latin six cases: the nominative, and the accusative (called in Old French the oblique). Masculine nouns seem to be perverse in this form, in that the nominative singular ends in -s, while the plural does not, and the oblique singular has a zero ending, while the plural ends in -s. As the two case system “decayed” into the no case system we have today (except for the pronoun declensions), the nominative was the form of most nouns that disappeared. Not always, however, and those names that retain the final -s are the last survivors of the Latin case: Charles, Georges, Gilles are three boys’ given names that retain -s – and note that in two of these cases so does modern English (Charles and Giles). Many place names also retain this final -s, and in English we have kept more, it seems, than the French themselves. Marseilles (English) or Marseille (modern French) is just that. Until at least the Second World War, Marseilles was pronounced /mɑːr’seɪlz/ in English, and the final -s (or /z/ sound) was dropped when we English started to realise that the French don’t say it that way – just like the change in Lyon.
Athel makes one very common slip in his lists of names: while Algiers is spelt with final -s in English, Tangier is not. In French, neither name has a final -s.