Intuitively I know it's wrong, but having done some research I didn't find any rule that would directly prohibit usage of an article before a number in a sentence like that. I understand that "a/an" in them and of themselves mean "one", but there must be some rule..

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    I have a one-son family. It's fine in that context. Commented May 29, 2020 at 20:52
  • 2
    hmm, I think one-son is an adjective here, so yeah, it does work
    – Leroy
    Commented May 29, 2020 at 21:11
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    A less technical way of explaining why it is incorrect: generally, we avoid superflous phrases in English. E.g. you can say 'more large' or 'larger', but 'more larger' is incorrect. 'I have a one son' has the same problem: 'a' and 'one' essentially have the same meaning in this context.
    – Joe
    Commented May 30, 2020 at 8:36

3 Answers 3


Let's look at the possible uses of a/an/some.

They are used when you talk about a countable* thing, and you're specifying how many of them you are talking about.

  • I have a son = I have one son (I may also have more sons, besides this one, but I am talking about this one for now)
  • There are some problems with your approach = There is more than one problem with your approach
  • An apple fell on my head = One apple fell on my head (perhaps another one fell later, but I'm talking about this one for now)

The reason I have a one son is invalid is because one son is not countable. It doesn't make sense for there to be two one sons, or even one one son.

(In response to comments on the question - I have a one son family is valid, because one son family is countable; "there are ten one son families living in this block of flats".

Another interesting case is I rolled a one [when rolling dice]. This is also valid - if you had 5 dice, you could say: I rolled five ones.)

* To be precise, 'a/an' are used before countable things, while 'some' can also be used before quantifiable things, like water; with 'some' you can state either 'how much' or 'how many'.

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    I don't think this is correct. While your examples are right, "one son" is not uncountable in a grammatical sense. In English grammar we talk about countable and uncountable to differentiate nouns that can be used with certain types of determines. An example of an uncountable noun is "water" as in "some water". If "one son" is uncountable in the same sense, then "some one son" would be valid. Commented May 30, 2020 at 18:43
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    It's true that you can't say "a water", but "some water" is representing a quantity of water. Perhaps the right term, then, would be "quantifiable" rather than "countable"? Commented May 30, 2020 at 19:49
  • In case of the dice, the "one" is not a count but a noun.
    – Klaws
    Commented May 31, 2020 at 8:15
  • Yes - a countable noun. It makes sense to roll one one, five ones, no ones etc. Commented May 31, 2020 at 11:46

The word that describes the function of "one" or "a" in this context is determiner (Wikipedia). Determiners are different from adjective, and always come before adjectives.

From the above:

Determiners may be subcategorized as predeterminers, central determiners and postdeterminers, based on the order in which they can occur. For example, "all my many very young children" uses one of each. "My all many very young children" is not grammatically correct because a central determiner cannot precede a predeterminer.

Encyclopedia.com states that articles, ("a" or "the"), along with possessives ("my", "his") and demonstratives ("this", "that") are central determiners. Quantifiers such as "one", "two", and "many" are postdeterminers. So "I have a one son" does follow this rule.

I can't find the specific rule that "I have a one son" breaks, but if you look into the topic there are many such rules, some of them very specific, so we can safely assume this is its own rule:

The indefinite article "a" cannot be followed by a cardinal number (such as "one") as a postdeterminer

Note that this doesn't include "one" as a noun such as "I have a one of hearts" (referring to playing cards), or part of an adjective phrase, "I have a one ton truck".

Note that this rule doesn't seem to apply to any other central determiner. "I have my one son. I have the one son. I have some two sons."

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    Is the definite article a central determiner? Sentences of the form "I have the one son" are quite common in British English colloquial speech although "I have a one son" is never used even colloquially except, possibly, in error. A similar construction "I have just the one son" is also common. Is it correct or merely colloquial?
    – BoldBen
    Commented May 29, 2020 at 21:09
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    In "I have a one-ton automobile", "one-ton" is an adjective phrase and not a determiner. Commented May 29, 2020 at 21:41
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    Sorry, but this is competely wrong. The word the IS a central determiner. It is the most famous central determiner, and the one usually given as an example of what a central determiner is. The numeral one is NOT a central determiner, just like other numerals like two etc. Commented May 29, 2020 at 23:26
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    @RrockCj It is correct. What they're saying is that the explanation is wrong.
    – Moyli
    Commented May 30, 2020 at 7:11
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    @RrockCj "I have the one son" is another way of saying "I have just one son" or "I have exactly one son". At first sight it is also synonymous with "I have only one son" but it is not usually quite the same as that as the statement is rather more positive in feel.
    – BoldBen
    Commented May 30, 2020 at 23:57

The simplest explanation would be that a/an = one, and thus there is redundancy.

The determiner theory would not explain "I have the one son.". Here "one" is an adjective = single or as OED says

II. Emphatic uses. 3.a. Designating exactly one, as opposed to two or more; a single ——. Frequently in negative contexts. Also preceded by a determiner such as any, no, some, the, this.

2002 Observer 28 July i. 24/3 A fine pub game in which you have to find the one London tube station that doesn't contain any letters from the word mackerel.

  • 13
    I'm don't think the redundancy is the problem, because "I have a single son" is perfectly acceptable. I think Alex Walker's answer about "one son" not being countable is the most plausible.
    – Paul
    Commented May 30, 2020 at 18:08
  • Redundancy might be part of the problem, depending on what was meant by the phrase. "I have only one son" would still retain some ambiguity, as it might or might not exclude children who are not sons. I'd rephrase as "I have an only son" if it were meant to exclude other children.
    – hardmath
    Commented May 31, 2020 at 15:46
  • I think the question is not whether son is countable but whether one son is countable. Though it's true that it's also not a mass noun, so I guess you're right there. Still, I think it's more down to the "counter" slot already being filled. Even ignoring the logical problem, you can't say, "I have a five sons" or even "I have some five sons" (in the sense of "some" meaning "some amount of" rather than "approximately").
    – Paul
    Commented Jun 2, 2020 at 20:49
  • @Paul "I have a single son" - one son, or a son who is not married?
    – Greybeard
    Commented Nov 28, 2023 at 17:20

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