It was one of those things that I hadn't noticed too much . . . until someone pointed it out. Turns out that yep, people do use this one wrong, and with somewhat alarming frequency.
Here's how you use "let alone":
"Let alone" ups the stakes. Generally, the speaker/writer is referring to two things, one much bigger/worse/awful/awesome than the other.
Rules of thumb:
(1) The smaller "less wow" item is first.
(2) The item with the biggest "wow" factor is mentioned second, after "let alone."
And that is where people make the mistake: by putting "let alone" next to the lesser item.
Example #1 (courtesy TJ)
Five-year-old Timmy asked for a pet, but we aren't getting a pony, let a lone a dog.
Here, the sentence implies that a pony is a common pet, while a dog (a DOG!) is something totally out of the realm of possibility, a ridiculous idea.
Five-year-old Timmy asked for a pet, but we aren't getting a dog [lesser pet!], let a lone a pony [the WOW pet!].
Example #2 (courtesy Sarah)
I've never been to London, let alone Salt Lake City.
For most of my readers (likely Utahns) and even those throughout the U.S. and the world, Salt Lake City is by far the lesser "wow" of the two cities. For starters, SLC, as an actual settled city (not counting Native Americans who may have lived in the area) doesn't even have 200 years of history yet, while London has hundreds and hundreds of years' worth.
I've never been to Salt Lake City [the lesser city, assuming you're big about history, art, literature, etc.], let alone London [the holy grail for writers and English major nerds like me, ergo the WOW city to visit].
As with many of my Word Nerd Wednesday posts, this issue probably a lot of readers wondering how anyone could get that wrong. Good!
For those few people who didn't know the correct way of using "let alone," now you can go forth and use it properly over Thanksgiving dinner, knowing that the word nerd at your table won't choke on a turkey bone.