Some of our best questions come from our listeners.
At our Issues and Ale event a few weeks back, we got a usage question that we've never even considered. An audience member wanted to know whether it's informal or incorrect to say "60 miles an hour" instead of "60 miles per hour."
What's interesting is that historically, people worried that "per" was informal and felt that "an" or "a" should be used instead.
Now the question has been turned on its head.
Concern over the "per" construction can be traced back to the early 19th century. Since "per" is borrowed from French and Latin, the thought was that it shouldn't be considered English.
"Per diem" and other sayings where the whole phrase was borrowed got a pass, but something like "per hour" was out. There was also worry that "per" sounded too much like legalese, as "Garner's Modern English Usage" puts it.
When it comes to mileage specifically, "miles an hour" was more common until about the 1950s. At that point, we see the switch and "miles per hour" takes over.
Today, we're quite comfortable with "per hour" or "per day", so now it's "an hour" or "a day" that get called into question for being too informal.
For the most part, these two constructions are interchangeable. However, there are some cases where you probably tend to stick to one or the other. "Per unit cost" and the saying "a dime a dozen" both come to mind.
Can you think of more? Let us know at email@example.com or firstname.lastname@example.org.