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I am a native speaker of AmE. I understand when and where to use their vs theirs, etc. etc. (i.e. Don't migrate this to ELL!). I've searched the site and google, and I have not quite seen an answer to my question.

Etymonline describes the word theirs as:

possessive pronoun, "their own," early 14c., from their + possessive -s, on analogy of his, etc. In form, a double possessive.

And, their:

plural possessive pronoun, c. 1200, from Old Norse þierra "of them," genitive of plural personal and demonstrative pronoun þeir "they" (see they). Replaced Old English hiera. As an adjective from late 14c. Use with singular objects, scorned by grammarians, is attested from c. 1300, and OED quotes this in Fielding, Goldsmith, Sydney Smith, and Thackeray. Theirs (c. 1300) is a double possessive. Alternative form theirn (1836) is attested in Midlands and southern dialect in U.K. and the Ozarks region of the U.S. Emphasis mine

(Parenthetical question, what do they mean by use with singular objects in this case?)

The entires for our and ours are similar.

Why does English have a double possessive pronoun? And why does modifying it thusly change its usage? Singular-plural possessive pronoun - possessive adjective; double-plural possessive pronoun - possessive pronoun?

Theirs is used when there is not a following noun, but, I don't understand why a double possessive would be used in this way.

The book is theirs.

That is their book.

We cannot say:

The book is their.

nor

That is theirs book.

But, for the life of me, I cannot figure out why making it a double possessive would make this happen!

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    Nothing does. Pieces of grammar like pronouns get misshapen in the gears as they go around, billions of times a day, year after year. You can't "explain" why one pronoun takes one form and a different one takes another, any more than you can "explain" why two fish have different sizes. Variation is the norm in evolution; it's consistency that's suspicious. – John Lawler Oct 5 '19 at 16:48
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    @JohnLawler Perhaps I'm just seeking order in the universe. – David M Oct 5 '19 at 16:53
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    Because pronouns don't behave like nouns? – John Lawler Oct 5 '19 at 17:07
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    Incidentally, theirs and ours aren't "double possessive pronouns". They're possessive pronouns, period. Our and their are the adjective form, but they take -s as pronouns. This is true for all personal pronouns except my and thy, which take -n from the old pre-vocalic variants mine and thine (like the eyes you should drink to me only with). The -s isn't possessive any more than the -n is. It's just a different paradigm. – John Lawler Oct 5 '19 at 17:11
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    As for following rules, it's the same as anything else. It doesn't follow simple rules, but when you look close enough you see patterns. But language rules are complicated by the fact that our culture doesn't look at language clearly. We think it's all letters and words, so sounds and constituents don't get noticed; but language rules work only for sounds and constituents, so it's kind of like trying to paint without noticing color. – John Lawler Oct 5 '19 at 17:14
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I don't know why etymonline calls theirs 'a double possessive', but it's not.

The unfortunate terminology of 'the double possessive', aka 'the double genitive', is not due to the pronoun theirs itself but to the common construction like a friend of theirs where traditional grammar treats the preposition of as another possessive on top of the possessive pronoun theirs.

So in a construction that doesn't contain of, theirs itself is no double possessive:

The book is theirs.

This example of yours, for example, doesn't contain of, so there's only one possessive, the possessive pronoun theirs, which means their book. (Note the subject of the first clause does contain of, so you can call it a double possessive.)

Now, some grammarians don't like the term 'the double possessive/genitive' even for constructions like a friend of theirs.

The Cambridge Grammar of the English Language (Page 468), for example, treats She's a friend of Kim's not as a double possessive/genitive but as a oblique genitive:

...we do not regard of as a genitive case marker, and hence there is only one genitive here, not two.

As for the distinction between their and theirs, CGEL classifies the former as a dependent genitive (possessive) and the latter as an independent genitive (possessive), which easily explains why these don't work:

*The book is their.

*That is theirs book.

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  • Thanks, that's actually quite helpful! – David M Oct 5 '19 at 17:31
  • Etymonline's wording is correct, and explains what it means: "In form, a double possessive". It just means that the form "theirs" was created by combining the possessive word their and the possessive suffix -s. Likewise, "children" can be described as a "double plural" in form (but obviously not as a "double plural" in any syntactic sense) because it was created by adding the plural suffix -en to the plural form childer. The CGEL's terms "dependent" and "independent" are of course appropriate for the purposes of synchronic description of the grammar of English. – herisson Oct 6 '19 at 8:55
  • @sumelic Forgive me about my total ignorance about "early 14c. English" discussed there, but did it use "-s" without an apostrophe as the possessive suffix back then? – JK2 Oct 6 '19 at 10:14
  • I don't know the history very well, but the use of the apostrophe in the genitive is complicated and has not consistently followed the same usage as modern English. Related questions: When did it become incorrect to use apostrophes with possessive pronouns?, Origins of possessive pronouns, Why is an apostrophe used in the genitive “-’s”? – herisson Oct 6 '19 at 10:54
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    I wonder if this is a generalization of a transformation needed for "her", which can be an objective pronoun as well (as in "I went with her"). We use the -s suffix to distinguish "X is her" (meaning X is the same as her) from "X is hers" (meaning X belongs to her). But we don't need that for the masculine form, which has objective "him" versus possessive "his". – Barmar Oct 7 '19 at 19:17

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