English: A vs. An

The rule for using "a" versus "an" in English depends not on whether a word's first letter is a vowel, but on whether its first sound is a vowel—making words like "unicorn" (consonant Y sound) use "a" and "hour" (vowel OW sound) use "an." An analysis of 32,455 words found that only 129 required these exceptions to the written-letter rule, suggesting most words follow predictable patterns that could be easily programmed into text generation functions.