When do we use One's / their ?

One must keep one's promise. I know this is correct as it is a well known phrase / quote.

But what about this sentence ?

One should exercise their right to vote.

Will we use one's here too ?

Is it that if subject is One then the pronoun will be always one's or it can be their too ?

  • In BrE the pronoun "one" is always used with "one's" and "oneself"/"one's self". In AmE there is a tradition of using "one" with "his", and hence (presumably) "their".
    – rjpond
    Oct 11, 2017 at 7:12

1 Answer 1


In colloquial English, we have a habit of using singular they. Grammatically, it's not advised (due to modern English using "you"), but it's become the norm to use "they" as a subject for any singular subject.

You can use either one or their in your example above.

One should exercise one's right to vote.

works just as well as

One should exercise their right to vote.

The former is more proper and is grammatically correct. One's always works as a possessive form for one. The latter is understood in spoken English, and is generally more widely used.

  • While I find singular they distracting, and take pains to avoid it, that's not the same as calling it incorrect. It's arguably no more incorrect than plural youye is the proper plural second person pronoun.
    – choster
    Jun 2, 2017 at 0:06
  • @choster Thanks! I suppose you're right. I revised my answer. :) Jun 2, 2017 at 22:01
  • Just to note, it's actually the singular you that was (once) grammatically incorrect. Ye was the plural subject pronoun, and you was the plural object pronoun. The corresponding singular pronouns were thou and thee.
    – PMV
    Jul 23, 2017 at 1:08
  • @PMV That's true. I did note in my answer that I was referring to modern English, which I supposed meant that, colloquially, we no longer use archaic terms like "ye" and "thou" and "thee" (except in specific circumstances, of course). It's become proper to use "you" for both singular and plural, but it's a good historical distinction to point out the word's former singular-only usage. :) Jul 25, 2017 at 18:05

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