1. My brother's friend's opinions
  1. My brothers' friend's opinions

Now, let us suppose that the brother in "1" is a brother-in-law and that all the brothers in "2" are brothers-in-law.

How do we rewrite sentences 1 and 2 in a possessive case?

I have done some preliminary research and found that when in-laws becomes possessive, brothers-in-law is written brother-in-law's. So, I would conclude that we should rewrite sentences 1 and 2 in the same way, as follows.

  1. My brother-in-law's friend's opinions.
  1. My brother-in-law's friend's opinions.

But, if this is the case, how can we distinguish the two cases?

4 Answers 4


So let's start with the singular 'brother-in-law', which is perfectly clear. If you have a single brother-in-law and he possesses something, this is written as:

My brother-in-law's cooking skills are excellent.

If you have more than one brother-in-law (no possession) you would write:

My brothers-in-law are all brunettes.

This is because when pluralizing a compound noun, we always add the 's' to the most 'important' word. The fact that they are brothers is most important, so it gets the 's'. This is the same for "mothers-in-law", "fathers-in-law", etc.

If you have more than one brother-in-law and they all own something:

My brothers-in-law's restaurant is the best in town!

Confirmation of this final construction can be found at grammarbook.com:

Rule 7

If the compound noun is plural, form the plural first and then use the apostrophe.

Example: my two brothers-in-law's hats

  • Wendi, I was confused by "The Cambridge Guide to English Usage", which excludes that "brothers-in-law's" is correct. In fact in that book it is said "But when in-laws become possessive, the forms are fully English: brother-in-law's, father-in-law's etc." (the mentioned forms are brothers-in-law, fathers-in-law etc.)
    – user114
    Mar 31, 2013 at 18:38
  • It's interesting that you say "My brother is a brunette". I would always go with "My brother has brown hair", since personally I've always associated "brunette" as a description of a woman's hair, not of a man's. I wonder if anyone else does this.
    – Matt
    Mar 31, 2013 at 18:42
  • 1
    @Carlo_R. Hmm. That doesn't sound right to me, because as you say in the question, you have no way of knowing it's plural, which doesn't make any sense. If you have more than one you have to state that it's plural somehow. (I don't understand the quoted phrase "the forms are fully English" either.) Hm. I will admit that "brothers-in-law's" sounds ridiculous to say. I'd prefer to say "The restaurant my brothers-in-law own is the best in town", for example, and avoid the issue altogether as it's likely to be confusing in speech. Does the book offer any explanation for it's position?
    – WendiKidd
    Mar 31, 2013 at 18:43
  • 4
    @Carlo_R. Oh definitely! Just because a misunderstanding of what the Cambridge Guide said caused your confusion doesn't mean your question wasn't real. And this is a very good reference to have for other learners with the same question :)
    – WendiKidd
    Mar 31, 2013 at 19:42
  • 2
    The quote from CGEL is unclear because 'in-laws' is a common way to refer to a number of them wihthou specifying which particular relation, mothers-in-law, brothers-in-law, some mix, etc.
    – Mitch
    Apr 1, 2013 at 2:37

This debate seems to rely on a couple of printed authorities (like the Cambridge Guide), but this construction is so low frequency that most grammars donʼt have any information on it. Few native speakers ever need to use it, so intuitions are hard to access.

The comments so far ignore the fact that syntax is not "flat"; grammatical units are grouped into hierarchical units. The plural of nouns belongs to the simple category of the noun, but the genitive/possessive belongs to the entire noun phrase, as proven by phrases like "the queen of England's crown" (not *the queen's of England crown): in [[the queen of England]s crown], the possessive S belongs to the phrase "the queen of England".

So the plural of "brother-in-law" (at least in the standard language) is "brothers-in-law", since the plural goes on the bare noun. The possessive cannot be *brother's-in-law; it has to be "brother-in-law's", and that is what native speakers say ("We went to my brother-in-law's house").

By this logic, the plural possessive should be "brothers-in-law's" (no matter what any guide says!), but at least where I come from, the colloquial language resolves it as "brother-in-laws'". We tend not to non-standard plurals (e.g. two brother-in-laws, two attorney generals). Let the purists cringe, but it's a more natural, "English" solution.

The readers of this post should decide who they trust more--a pronouncement from a guide on a low-frequency construction, or the intuitions of millions of native speakers of English. What would most people produce and/or comprehend?


In a Google search on "brother-in-law plural possessive" this afternoon I found that nine out of ten pages (including this one) recommend "brothers-in-law's".

The readers of this post should decide who they trust more -- the intuition of a representative native speaker of colloquial English, or a search on millions of pages of written English. Perhaps it depends whether you're speaking or writing. If you're speaking, people will understand you even if you make mistakes. If you're writing, it's hard to argue against the unambiguity of "brothers-in-law's".

  • 2
    I think it's rather telling that in the billions of pages that Google has access to, apparently only 10 pages even mention it. This would suggest that written constructions are rare, and I would question the authority of these pages - are they more pages that cite native-speaker intuition as their source? An ngrams search is a much better representation (which, incidentally, finds brother-in-laws's to be more common). As you rightly state, however, these corpora generally only index written language (which the OP asked about) - spoken language is another matter.
    – jimsug
    Dec 9, 2014 at 14:55

It goes back to Old English and it's germanic roots. So it's a survivor of an older sentence structure to pluralize 'Brother in law' to 'brothers in law'. Of course the 's possessive is a much later addition to the language. Those guys on the beach in Normandy waiting for William the Duke of Normandy's army would have (translating) said "the army of Duke William" and "the restaurant of my brothers in law". However this old construct has survived the 'technically' correct form for the posessive is indeed "brothers in law's". In practice, it depends on environment and company. Use whatever gets you less funny looks and correction in the current situation.

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