"Primmer" is the older pronunciation, but how you say it depends on where you live.
A couple of weeks ago when I did the show on Esperanto, I mentioned that Dr. Zamenhof wrote a primer for his new international language, and a listener named Richard wrote in to suggest that I should have pronounced that word “primer,” and he was right. I was not aware of the difference—I just thought “primer” was a kind of pretentious way of pronouncing the word, but it turns out that a primer and a "primmer" are two different things, especially in American English.
“Primer” with a short I, pronounced like “primmer” and meaning an introductory book, is the older word, going all the way back to the 1300s. It described both a schoolbook and a prayer book since at the time reading was taught from prayer books, according to Etymonline. It came from the Latin word “primus,” which meant “first,” and which doesn’t sound like either modern day pronunciation. Think of a primer as the first book you’d read on a subject, the thing that gets you started.
In American English, we use the "primmer" pronunciation for an introductory book.
“Primer” with a long I, rhyming with "climber," came from an extended meaning of that same Latin word meaning “first.” In the late 1600s, people started using it to describe the first coating of paint or dye you apply to something. (I have used a lot of primer in my time.) And then later, in the early 1800s, people started using it to mean, among other things, something you use to set off an explosive charge, like a cap.
The same root also gives us the words “primary” and “primo” as in the slang word for “first-class.”
That is some primo chocolate cake!
The Oxford English Dictionary says that “primer” (rhyming with “skimmer”) is the original pronunciation. For reasons I couldn’t find, the “primer” (rhyming with “climber”) pronunciation also arose, and then the two pronunciations came to be used differently in American and British English.
'Primer' in American English and British English
In American English, it’s clear cut: A primer (pronounced "primmer") is a schoolbook, and a primer (rhyming with "climber") is the first coat of something, like the white stuff you roll onto drywall before you put on your real paint, or an explosive cap.
In British English, the pronunciation that is closer to “primer” (rhyming with “climber”) is usually used for all the meanings.
But…I am not British, so thank you to Richard for making me aware of the different pronunciations and their meanings.
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Mignon Fogarty is Grammar Girl and the founder of Quick and Dirty Tips. Check out her New York Times bestseller, "Grammar Girl’s Quick and Dirty Tips for Better Writing."