A correspondent writes to ask why millionaire is spelt with only one n, whereas questionnaire has two.
It's the other way round, really. Why does questionnaire have two, when millionaire has one? All these -aire words came from French, where they had a double n spelling. When they arrived in English, there was usually an initial period of uncertainty: both spellings can be found in the early citations in the OED. We find millionnaire alongside millionaire in the early 19th century; and a few decades later, the same alternation with billionaire. The English norm was to simplify to a single consonant, which is what we find with commissionaire and concessionaire, presumably following the pattern of the much earlier borrowing debonair, and also doctrinaire, which are recorded for the most part with a single n. Dr Johnson gives only debonair in his dictionary, and that decision will have exercised considerable influence.
Questionnaire is the exception. Why didn't it follow the normal pattern? H W Fowler insisted that it should lose the extra n in his Dictionary of Modern English Usage (1926), but his view was ignored. The OED has examples from 1901, but no instance of a single n, despite the huge potential influence of questioned, questionably, and the like. Perhaps the purist focus on the word caused people to pay more attention to the French spelling than would otherwise have been the case. It's a puzzle - but then, linguistic exceptions usually are.