How to spell questionnaire
questionnaire
Not questionaire.
The trap
The word questionnaire has two Ns, and questionaire keeps the single N of question. The longer word does not inherit the shorter one's spelling; it has its own.
Doubled letters are part of how a word is built, not decoration. The reason behind a double varies from word to word. Often it is an added ending, sometimes the word's history, so the double is learned along with the word.
What it means
noun. A set of printed questions for a survey.
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Play today's Daily FiveSpellingIQ is a skill rating for spelling ability, not a clinical or psychometric IQ measure. American English is canonical; common variants are accepted.