Both stationery (n.), meaning writing paper, and stationary (adj.), meaning not moving, go back to the same source that gives us the noun station (a stopping place; a place where someone or something stands).
In the Middle Ages a stationer was a tradesman who had a shop, as opposed to one who carried his stock around with him. Originally the word was used in a general sense to denote any shopkeeper, but came to be associated with booksellers in particular. Finally, stationer came to mean someone who sells writing materials, i.e., stationery.
TIP: Stationery, like paper, is spelled with –er. A parked car is stationary. Like park, the adjective stationary is spelled with –ar.
This seems to be an especially endangered spelling. I have a local real estate map that spells cemetery incorrectly dozens of times. I’ve even seen an official cemetery sign with the incorrect spelling.
TIP: Knowing that cemetery comes into English from French cimetière may help writers recall the correct –er spelling.
Browse the “pets for sale” in any classified section and you will see the word for the dog spelled “sheperd, “shepard,” and “shephard.” At least two of these misspellings for the dog’s name are common spellings for family names. For the dog, however, the spelling is shepherd.
TIP: The dog was originally bred to help in the herding of sheep or cattle. The word shepherd comes from sheepherd. A shepherd herds sheep. Think “herd.”