English: a vs an

English: a vs an

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Blog post:
16 Sep 2026

In English, there is an “indefinite” article a that can go before a word. For example, a raccoon. But for some words, we use an. For example, an apple.

When procedurally generating text, I want a function
a_or_an("apple") that tells me which article to use. That seems
like it’d be easy. We can check the first letter to see if it’s a
vowel. But that would mean we output an unicorn, not a unicorn.

The actual rule is not whether the written word starts with a
vowel letter, but whether the spoken word starts with a vowel
sound. The word unicorn starts with vowel letter (u) but a
consonant sound (Y). The word hour starts with a consonant letter
(h) but a vowel sound (OW).

Visualization showing whether the first two letters of a word are enough to determine whether it should have “a” or “an”

I was curious how often these exceptions occurred, and whether they can be grouped together, so I spent a day looking at the data and building some visualizations and wrote up the results.
I was surprised that only 129 of the 32,455 words in my list needed exceptions.

[LLM note: I did not use LLMs to write any of this code, but in hindsight, I should have. This is one-off code to answer a question. It doesn’t need to be clean or maintainable. It only needs to be correct. I would’ve spent more time on the trie simplification algorithm and less time on parsing cmudict and re-learning d3.js.]

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Created 16 Sep 2026;
Last modified: 16 Sep 2026

The text explores the nuanced rules governing the usage of the indefinite articles "a" and "an" in English, moving beyond the superficial rule of starting letters to focus on phonetic sounds. The author posits that the determination of whether to use "a" or "an" is based on whether the spoken word begins with a vowel sound or a consonant sound, rather than the written letter. This distinction introduces exceptions to a simple letter-based rule, exemplified by words such as "unicorn," which begins with the vowel letter 'u' but is pronounced with a consonant sound, and "hour," which begins with a consonant letter 'h' but is pronounced with a vowel sound.

The author developed a concept for a function, a_or_an("apple"), intended for use in procedurally generating text, to address this phonetic complexity. The process of establishing this rule involved an empirical investigation into exceptions, where the author analyzed a list of thirty-two thousand four hundred and fifty-five words. This analysis revealed that only one hundred twenty-nine of these words required exceptions to the general rule, suggesting the existence of significant, yet relatively small, deviations from the simple vowel/consonant correlation. To visualize these findings and understand the distribution of these exceptions, the author created data visualizations. The author also provided a reflective note regarding the development process, acknowledging that the generated code was created without the use of large language models, emphasizing that the primary goal was correctness, and noting areas where further time could be allocated to more complex algorithmic components such as trie simplification and parsing.