Is it stationary or stationery? Does it matter? Does anyone really notice? Well, yes: of course it does, and of course they do.
Stationary (-ary) means not moving. Stationery (-ery) means writing paper and envelopes.
If someone confuses the two and makes an error, it’s more often by using the -ary form (not moving) when the -ery form (paper) is meant. The error happens in the other direction too, of course, but less frequently.
Ordinarily, if I come across this error in an edited publication, I’ll cringe for a moment, but then move on. Once it’s out there, it’s not my problem. However, I recently came across a serious repetition of the error and it was painful to read, so a quick post seemed in order.
The publication was a regularly published — and presumably professionally edited — journal of fiction, and one of the short stories in the issue involved a stationery shop (it wasn’t critical to the plot, but it was used repeatedly). Over the course of the story, the word stationery was used perhaps six to ten times. And in every single case it was misspelled as “stationary!”
A few paragraphs are in order on the origin of these two words, stationary and stationery. Some of this is speculative, but I’ve pulled it from the usual sources (especially OED); what I’m giving you seems a reasonably good etymology and history, although I haven’t provided references.
In the distant past, these two words sprang from the same source: the Latin stationarius, which originally meant something belonging to a detachment of soldiers, including one of the soldiers himself. It eventually came to mean someone associated with a fixed post, and frequently someone controlled by (and sworn to obey) an academic authority. This use, the stationer (one on a station), is the oldest in English and entered by the early 14th century. This and the stationary (immobile) use might have been influenced by similar words (also derived from the Latin stationarius) with similar meanings that passed through French, Spanish, Portuguese, and Italian.
Stationer eventually came to mean a bookseller. The transformation happened something like this: first the stationer was an occupier of a specific post (a person at a station); then the stationer at that post became a licensed position holder; that position holder, the stationer, was licensed to trade in certain items, many of which were associated with the (frequently) academic nature of the post — books, paper, and so on; eventually, any dealer in similar items was given the same label of stationer. (While my description here is fast and loose, the sources support this general idea.)
The later English stationarius (and then stationer), a person with a job that put him on a permanent station, seems to have been analogous to the porters still sometimes employed at residential colleges (especially in the UK) — a sort of combination concierge, warden, house mother, butler, etc. — but also with some commercial responsibilities.
Evolving from stationer (one on a station, not the bookseller), the adjective stationary (fixed, not moving) was in use by the middle 15th century. The noun stationery was in use by 1727, and was used much in the sense it’s used today: things associated with a stationer, or a stationer‘s wares. The spelling was still not consistently distinct, although the -ery form was established as dominant by the last quarter of the 18th century. It’s a reasonable guess that the -ery form was a deliberate spelling change, meant to distinguish the two words and keep one closer to the stationer (stationarius having died out, and stationary having strongly established itself as an adjective). Spelling it with the -ery form was probably initially a bit of an affectation, but it’s turned into a useful distinction.
On the whole, I suspect this is one of those language issues that follows the pattern of Fowler’s observation on split infinitives, namely that there are five categories of people when it comes to usage considerations: those who neither know nor care, those who don’t know but care very much, those who know and condemn, those who know and approve, and those who know and distinguish. The vast majority probably don’t know or care, and won’t notice if you misuse the -ery or -ary form. But if you’ve read this far, or even simply come to this page looking for information, you’re already in one of the other categories. You care, and that’s a good thing.
That’s really all you need to know. Your stationery can be stationary, but try your best to use the correct forms: -ary if you’re writing about not moving, -ery if you’re writing about paper.