The difference between “less” and “fewer” was one of the first issues to come on my radar as a beginning copy editor. Unfortunately, I learned it wrong, as I found out a few years later. Even more unfortunately, I then proceeded to learn it wrong again. It took a few more years to discover my second mistake.

Luckily, none of my misconceptions resulted in any editing errors. In fact, my confusion would have been completely harmless had I not run around telling people they were wrong because they weren’t doing it the way I learned.

My first lesson on the subject, naturally, made reference to grocery store express lanes. It went like this: The signs reading “10 Items or Less” are wrong and should be “10 Items or Fewer.” This is rooted in a good idea, but overstated to the point of being, well, wrong.

The reason it should be “10 Items or Fewer,” I was told, was that “less” applies to quantities while “fewer” applies to countable things. That is, “less” modifies what are called mass nouns — stuff like milk, money, flour, wood and time — whereas “fewer” modifies what are called count nouns — stuff like apples, dollars, wood beams and minutes.

So while you could have less milk, less money, less flour, less wood and less time, you’d have fewer apples, fewer dollars, fewer wood beams and fewer minutes.

Sometimes the difference is subtle: less money, fewer dollars. At its heart is the question of whether you’re emphasizing individual units or a lump sum. Some words can be either count nouns or mass nouns, depending on the speaker’s intent. Joe is cutting back on soda. Joe is cutting back on sodas.

So, according to this explanation, because “items” is a count noun, it takes “fewer”: 10 Items or Fewer.

This is partly true. Less does modify mass nouns and fewer does modify count nouns. But that doesn’t explain the real difference. Nor does it give you enough information to know whether “10 Items or Less” is really wrong.

The first revision to my understanding of “less” and “fewer” came when I was presented with the following scenario: Say you’re in the express lane and realize you have 11 items in your cart. You put your copy of People magazine back on the rack. Do you now have one fewer item? Or do you have one less item?

According to our mass-noun-vs.-count-noun rule, you would say “I have one fewer item now.” But that’s wrong, as your ear may have been hinting to you already. You have one less item.

In this situation our rule breaks down because it wasn’t really a rule in the first place — just a good guideline that works 99% of the time. The real issue isn’t about mass nouns vs. count nouns. It’s about singulars vs. plurals.

Less modifies singular things. Fewer modifies plural things. Mass nouns like milk and wood and courage get “less” not because they’re mass nouns but because they are singular. But count nouns have singular forms, too. “Item” is singular. “Items” is plural. That’s why you’d have “one less item” even as you had “fewer items.”

If you want to observe a distinction between those words, there it is. But be careful not to make my final mistake: confusing this idea with a true rule.

According to Webster’s New World College Dictionary, one of the definitions of “less” is “fewer.” They’re synonyms.

So, while perhaps not the best form, that grocery store express lane sign wasn’t really wrong in the first place. But I was.

JUNE CASAGRANDE is the author of “It Was the Best of Sentences, It Was the Worst of Sentences.” She can be reached at JuneTCN@aol.com.