Do you buy your writing paper in a store that sells stationary or stationery?
How to Remember It:
For one, consider the histories of these words.
Stationery comes from stationer, a word that in the 14th century referred to someone who sold books and papers. What the stationer sold eventually came to be referred to by the noun stationery ("materials for writing or typing" and "letter paper usually accompanied with matching envelopes").
Meanwhile, the adjective stationary has always been used to describe what is fixed, immobile, or static.
Here's another way to remember it: stationery is spelled with an "e," like the envelopes that often come with it.