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“A One” or “An One”: Understanding Correct Grammar

Both “a” and “an” are extremely common words in English that we use to modify nouns, functioning in a similar way to an adjective. The basic rule for using the articles “a” or “an” is that we use “a” before words beginning with a consonant and use “an” before words beginning with a vowel, so which would you place before “one”?

“A one” is correct because “one” starts with a “w” sound. In addition to the vowel rules regarding articles, we must also consider the sound at the beginning of the word. The situations where we would use an article before “one” are uncommon, and we most often use this when referring to adding a one to something.

This article will explore articles in the English language, as well as the rules governing the use of “a” and “an” before “one.” It will also explore the history of their usage and changing trends, as well as instances when we might use the phrase “a one.”

When to Use “A One”

We should now be able to agree that the correct article usage for “one” would be “a” rather than “an,” even though the word starts with a vowel. We pronounce the word “wun,” so it begins with a consonant sound.

But the question to consider now is when we would have the opportunity to refer to “a one”?

Numbers can play all sorts of roles in sentences, depending on the context, so there would be many opportunities to use “a one” in a sentence. For more information on the role of numbers in sentences, read “Are Numbers Adjectives?

Most often, numbers are determiners of quantity and serve to modify the noun of which they speak. The only time we could refer to “a one” on its own would be to refer to the number itself, as in the following examples.

You need to put a one below the four in that equation.

I picked a one from the bucket of plastic numbers.

There wouldn’t be much opportunity to use “a one” in this way, as it’s not something we regularly speak about. We would be more likely to use it when we join “one” with another word to create a compound word. Consider the following examples.

  • A one-sided argument
  • A one-word answer
  • A one-dimensional approach
  • A one-way road
  • A one-legged doll
  • A one-woman show

“One” can also be a pronoun, referring to people in general. Here we might say something like, “Scenes like this always move one.” In this context, and when using “one” as a pronoun, we would never use an article before it.

Using “A” or “An” Instead of “One”

There are times when “a” or “an” are interchangeable with “one.” Generally, if either is appropriate, you would use “one” if you want to emphasize the number and “a” or “an” if you are being less specific. Consider the sentences below.

Please, can I have a pint of beer?

Please, can I have one pint of beer?

She was away skiing for a week.

She was away skiing for one week.

If it’s necessary for clarity to emphasize a single one, then it’s better to use “one” rather than “a” or “an,” which can be less specific. However, in all other cases, it sounds more natural to use “a” or “an” rather than “one.”

When to Use “A” and When to Use “An”

Now we know that “a” or “an” is used only with singular count nouns where we don’t know the noun’s specific identity and are talking in general. But, we also need to know which one to use when.

The general rule of thumb is to use “a” before a consonant sound and “an” before a vowel sound, regardless of spelling. This rule is essential because you have to know how the word sounds before deciding whether to preced it with “a” or “an.”

It’s pretty simple when vowels sound like vowels and consonants sound like consonants, as in the examples below.

  • A boy
  • An ice cream
  • A trick
  • An idiot

However, it becomes a little trickier when you come across a vowel with a consonant sound or vice versa. Consider the following examples , where the words are spelled out phonetically to illustrate the point.

  • An honor (ah-ner)
  • A university (yoo-ne-ver-se-tee)
  • A user (yoo-zer)
  • An hour (our)
  • A one-way street (wun-way)

In these cases, it’s best to say the word out loud and ignore its spelling to decide which article to use.


The same rule applies when writing or speaking about acronyms or initialisms — you have to consider the sound of what you are saying. Therefore, you would say the following:

  • A US-based professor agreed with the report.
  • He followed an NEC recommendation.
  • That is an HTML document.

With an Adjective

If an adjective has modified the noun, then your choice of “a” or “an” will depend on that adjective’s opening sound. Take a look at the following examples to understand this.

She wore a green apron.CORRECT
She wore an green apron.INCORRECT
My shirt has an ink stain.CORRECT
My shirt has a ink stain.INCORRECT
This is an unusual idea.CORRECT
This is a unusual idea.INCORRECT
He has a one-eyed dog.CORRECT
He has an one-eyed dog.INCORRECT

Likewise, if an adverb follows the article, you will use “a” or “an,” depending on the word following immediately afterward. 

Exceptions to the Rule

There are always exceptions in English, and this rule has caused some disagreements among grammarians. This relates specifically to words that begin with “h.”

From the 18th century into the 19th, English speakers used a much softer “h” sound at the beginning of some words. If you look at texts from this time, you will often see reference to “an history” or “an historical.”

English speakers today, however, tend to speak with a much more distinct “h” sound at the beginning of such words, and it has therefore become common practice to refer to “a historical fact” or “a history of the world” (source). 

You may occasionally see text referring to “an historical,” and although it isn’t incorrect, it is becoming less common.

For further information on this and other style questions in English, refer to Dreyer’s English or The Oxford New Essential Dictionary. Both are available on Amazon and are invaluable resources. 

With “h” words, it depends on whether the “h” is pronounced or not, and this varies from word to word. Below is a list of some of the most common with the corresponding articles.

  • An hour
  • An honor
  • An herb
  • An heirloom
  • An heir
  • A humble person
  • A hysterectomy
  • A hereditary trait

An interesting one is “herb,” which is pronounced differently in American and British English. Where Americans say “erb,” British speakers will pronounce the “h” at the beginning. As a result, the article will be different for each of these.

The word was borrowed from the French herbe, which has a silent “h.” American pronunciation tends to approximate the sound of borrowed words, while British English tends to pronounce them according to spelling rules. 

Image by Gerd Altmann via Pixabay

What is an Article?

Articles are determiners, and, as such, they give more information about the noun they precede. They act similarly to adjectives, but they provide a reference to something already in context rather than just describing the noun.

You cannot eliminate an article from a sentence and have it still make sense, whereas an adjective is dispensable. 

There are three articles in the English language — “a,” “an,” and “the.” We use “the” for specific nouns or definite articles, and we use “a” or “an” for non-specific nouns or indefinite articles (source).

For example, if I said, “Let’s go to the park,” then I’m referring to a specific park. However, if I said “Let’s go to a park,” then I’m talking about any park.

Definite Article “The”

“The” signifies that the modified noun is definite, referring to a particular member of a group. We use it before nouns, singular or plural, where the noun is specific. Consider this example:

There is the man who stole my phone.

This article refers to a particular man, not just any man.

The doctor saved my mom’s life.

This article refers to a particular doctor and not just any doctor. We may not know his name, but it is still a specific individual.

I saw the alligator at the zoo.

This article refers to a specific alligator, possibly the only one at the zoo.

Indefinite Articles — “A” or “An”

“A” or “an” signifies that the modified noun is indefinite, referring to any member of a group. Consider the following examples:

I want a purse for Christmas 

The use of this article means this refers to any purse. We don’t know which purse because they haven’t chosen one yet.

Call a doctor!

This sentence refers to any doctor. We need any available doctor.

I saw an alligator at the zoo.

The speaker refers to an alligator, probably one of many, that I saw at the zoo and is non-specific about which alligator I saw.

We also use “a” or “an” to refer to a single one when there is more than one. The examples below illustrate this use.

I have two cats and a dog.

Please, may I have an apple?

Take a look at the sentences below, which illustrate how we use “the” to describe something specific and how we use “a” or “an” when we are speaking more generally.

An envelope was under my pillow.

The envelope was under my pillow.

My friend was bitten by a dog.

My friend was bitten by the dog.

Both of these examples show the difference between the definite and indefinite article — in both cases where the reader knows the noun’s identity and the writer uses “the,” it is clear that they refer to a specific envelope or dog.

When we don’t know the noun’s identity, and we use “a” or “an,” then we are talking about dogs or envelopes in general.

Using Articles with Count or Non-Count Nouns

Count nouns refer to items that we can count, while non-count nouns are those that we cannot count. Count nouns can be singular or plural, while non-count nouns are always singular (source).

We can use “the”  for both count and non-count nouns, while “a” or “an” can only be used for count nouns. Consider the examples below.

Count nouns:

  • I ate an apple.
  • She rented a house.
  • He opened the gate. (singular)
  • She peeled the bananas. (plural)

Non-count nouns:

  • Please pass the salt.
  • There was a storm over the water

The third category is nouns that receive no article at all. This is when we are speaking about things in general, and we do not use “the,” “a,” or “an.” Consider the examples below.

I never drink tea.

Waves are beautiful to watch.

In both sentences, we don’t give an article to either “tea” or “waves” because we are talking about all tea and all waves, so we don’t require an article to modify the noun. 

Image by Seven Shooter via Unsplash

Final Thoughts

Learning when to use “a” or “an” is mostly covered by learning the rules. But, it’s also important to know how to pronounce the word that comes immediately after so that you know which one to choose. If it feels awkward to say, then it’s probably the wrong choice. 

Most words are simple, but it’s best to say the word out loud and hear if you start with a soft vowel sound or a hard consonant sound whenever there’s a choice. If it’s the former, then choose “an,” and if it’s the latter, then “a” is the way to go. 

As you get more fluent, the choice of “a” or “an” will become more subconscious, and you will intuitively know which one to choose.