These envelopes and papers hold perfectly still! They don’t move at all!
I thought of what ought to be an easy way to remember the correct spelling of this word: these things are sold by a stationer, not by a stationar. (The one hitch to this mnemonic is that likely most people who spell it “stationary” also have no idea what a stationer is.)
Now I have a question. I’ll go ask Dictionary.com and be right back.
I’m back. And it turns out that, centuries ago, the word “stationer” came from the word stationary, because publishers, booksellers, and purveyors of writing goods had permanent shops rather than being itinerant vendors. So there you go! Now it should be easy to remember the correct spelling: just think of the origin of the word, then spell it differently!