In January, I took part in an interesting discussion on Twitter. Washington Post copyeditor Bill Walsh (@TheSlot) posted a headline:

Hole-in-the-walls: East, west, and downtown, 19 named

He asked, "Would you take your sister-in-laws to such a place?"

The question is how to pluralize a compound noun. The first examples were hyphenated, but as the conversation went on, noun phrases were added too, including:

attorney general
needle in a haystack
block and tackle

We know that to make a noun plural in most cases, we add –s or –es to the end of the word: spoons, boxes. But do you have attorneys general or attorney generals?

The General Rule

Usually you pluralize a compound noun or a noun phrase the same way you pluralize a noun: you add the plural prefix –s or –es to thenoun (irregular nouns notwithstanding). The trick in something like attorney general is to identify what's called the head noun. That is, the principal noun in the phrase.

An attorney general is the chief lawyer for a government. General is acting as adjective rather than a noun, describing the attorney's job. So if you have two such lawyers, you would have two attorneys general.

This holds true with your sister-in-law. In-law tells us more about the sisters: that they are related by marriage, not blood. Sister is the head noun, so when we pluralize we get sisters-in-law.

This can even work with some closed compound nouns, like passerby. The head noun here is passer, with by describing such a person. We're still going to add the plural suffix to the noun: passersby.

Plural Changes the Meaning

One phrase that stumped us in the original discussion was needle in a haystack. Needle is our head noun, so it seems our plural should be needles in a haystack. But that changes the meaning of the phrase; after all it's easier to find two or more needles in a haystack than just one. And looking for a needle in haystacks might be difficult, but it too changes the original meaning.

Needles in haystacks could work. Readers familiar with the original phrase will likely get the right meaning. It's not ideal, though, because the result is vague. You might be looking for lots of needles in each haystack, which is an easier task.

When pluralizing the noun phrase blurs or changes the original meaning, your best bet is to keep the phrase singular, rewriting the sentence if you have to.

Two Nouns of Equal Weight

Another problematic phrase is one with two nouns of equal weight, such as block and tackle. The term describes a pulley system used to lift heavy objects: one pulley called by its two chief parts.

Logically, we might pluralize both nouns, since both are of equal value: blocks and tackles. But this presents two problems.

The lesser problem is a possible confusion with block and tackle as sports terms. The New England Patriots might have many blocks and tackles to their credit, but did the New England patriots have a lot of pulleys in their barns during the Revolutionary War?

It's a problem of context. As long as the context makes clear that pulleys are the subject, we don't really have an issue.

The second problem, though, is a bigger concern.

Block and Tackles, Attorney Generals, and Other Oddities

Over time, terms like block and tackle become greater than the sum of their parts. The words are used together so often that we become accustomed to thinking of those parts as one unit. The result is that we put the suffix on the end of the unit. In the early patriots' barns there might have been several block and tackles.

The same thing happens with back-and-forth. You can have several back-and-forth discussions with your boss, or several back-and-forths. It even works for mixed drinks: as Ben Zimmer related in 2010, a grammar panel at the American Copy Editors Society conference decided that the best plural of the cocktail Captain on the Porch is Captain on the Porches.

This is how we get hole-in-the-walls, sister-in-laws, attorney generals, and others as well. But language changes over time and not all language users adapt to changes at the same time. So many of us will add two tablespoonfuls of sugar to a recipe, while others still add two tablespoonsful.

How Do You Choose?

First, check your dictionary. Dictionaries record not only plurals but also variations of plurals. Merriam-Webster Unabridged, for example, lists tablespoonfuls as the plural and tablespoonsful as a less common variation.

If your term or its plural isn't in your dictionary, stop to think about whether you understand the phrase as individual parts (add the suffix to the head noun) or as one term (at the suffix to the end of the unit).

Keep your audience in mind, too. In casual writing or with an audience familiar with the term, placing the plural suffix at the end of the unit may be fine. If you put the plural on the head noun, it may sound too formal.

On the other hand, if your audience isn't familiar with the term or is especially conservative about change, you may want to follow the rule more closely, talking about attorneys general rather than attorney generals.