1. A word or pronunciation that distinguishes people of one group or class from those of another.
2. a. A word or phrase identified with a particular group or cause; a catchword.
   b. A commonplace saying or idea.
3. A custom or practice that betrays one as an outsider.

Readers and writers, teachers and editors, are forever getting them mixed up with rules. How to tell a rule from a shibboleth? Rules usually further the cause of clarity: verbs should agree with their subjects in number; pronouns should agree with the nouns they refer to. Shibboleths often don’t. No surprise there: their main purpose isn’t to facilitate communication; it’s to separate those who know them from those who don’t.

To complicate matters even further, the language is continually evolving. New words are born. Meanings morph. Nouns get verbed and verbs get nouned. If you’re too far ahead of the pack in adopting a new usage, someone‘s not going to be happy about it.

If that’s not enough, we’ve also got such everyday confusables like ensure/insure/assure, affect/effect — and is it irrespective that’s OK and irregardless that’s verboten, or is it the other way round?

No wonder English starts to look like a minefield, even to native speakers who use it all the time.

Editors have been known to make it worse. Been there, done that. It’s an occupational hazard. An example:

As an apprentice editor, I was initiated into the mysteries of the which/that distinction. “That” was for restrictive (essential) clauses: “The sweater that I’m wearing was made by my mother.” (This implies that I have other sweaters and my mother probably didn’t make all of them.) “Which” was for non-restrictive clauses: “The house, which was built in 1850, has been in his family for decades.” (The building date is extra information. It doesn’t specify which house has been in his family for decades.)

Hoo boy, did I go wild or what. Anyone who hadn’t mastered the which/that distinction was an ignoramus. I got to look down my snoot at them. I got to educate them.

Then I learned that British English (BrE) was managing to get along quite nicely without the which/that distinction. BrE writers liberally used which” for restrictive clauses.  Their editors weren’t changing every “which” to “that.”

Wonder of wonders, I had no trouble understanding which clauses were restrictive and which weren’t.

By that time I’d so internalized the which/that distinction that it came naturally to me. This was an asset when I started copyediting for U.S. publishers, many of whom require copyeditors to change every restrictive “which” to “that.” Fortunately most writers won’t fight about this. Many have internalized the which/that distinction just the way I did. When editing the work of a BrE writer, I’ll generally stet the restrictive “which” and note it in my style sheet so the proofreader will realize that this was a conscious decision on my part, not a (gods forbid) mistake.

Another shibboleth is the widespread notion among U.S. copyeditors that “toward” is American English and “towards” is British English. They mechanically knock the “s” off every “towards” they come to. A few years back, Jonathon Owen, linguist, writer, and editor, did his master’s thesis on this very subject. As reported in his excellent blog, Arrant Pedantry, his research suggested that U.S. editors are creating the perception that “toward” is AmE and “towards” is BrE. For writers, it’s six of one, half dozen of the other. In edited manuscripts, however, “toward” overwhelmed “towards,” 90% to 10%.

In a recent online discussion, an assortment of editors took on the difference between “such as” and “like.” (If you haven’t heard of it, worry not: I’d been editing for 10 years before I was initiated into this particular mystery. Till then I thought “such as” was simply a more formal synonym for “like.”) According to those who observe the distinction, if I refer to “movies such as Lawrence of Arabia,” I am including Lawrence in the group. If I write “movies like Lawrence of Arabia,” I’m not.

Most of the editors participating in the discussion thought the such as/like distinction was a made-up “rule” — a shibboleth. I rarely use “such as”; when I use “like,” I’m not excluding the item(s) that follow from the group. I’ll wager that most writers do likewise, and — even more important — so do most readers. What this means is that if it’s important to know whether the item(s) are included or not, you better not rely on the such as/like distinction alone to get the message across. (The discussion suggested that readers of scientific literature were alert to the distinction, so if that’s your audience you’d best observe it.)

A caveat: English is riddled with sound-alike and look-alike words that don’t mean the same thing. These aren’t shibboleths. They facilitate communication. If you write or read, they’re worth learning. As an editor, I’m always on the lookout for them. The very capable author of a recent editing job consistently confused “imply” and “infer.” (A speaker implies that something is true. Her listeners may infer the truth from what she said.) I made the necessary changes and explained the difference to the author. He said he had a hard time keeping those two words straight.

Why does any of this matter? Here I turn to “Rules That Eat Your Brain,” by Geoffrey Pullum, linguist and frequent writer on English grammar and usage. “Zombie rules” are shibboleths by another name.

Though dead, they shamble mindlessly on. The worst thing about zombie rules, I believe, is not the pomposity of those advocating them, or the time-wasting character of the associated gotcha games, but the way they actually make people’s writing worse. They promote insecurity, and nervous people worrying about their language write worse than relaxed people enjoying their language.

If the language really were a minefield, what fool would venture out into it? Be brave. Write on.