The prefix "an" is used before words that begin with a vowel sound, including both long and short vowels. This includes numbers that are pronounced with a vowel sound, such as "one," "two," "three," and "eleven." For example, "an apple" and "an hour" both begin with vowel sounds. The use of "an" before numbers is consistent with the general rule of using "an" before words that start with a vowel sound, regardless of whether the word is a number or not.

Data from the American English corpus, such as the Corpus of Contemporary American English (COCA), shows that the prefix "an" is commonly used before numbers that are pronounced with a vowel sound. For instance, in the COCA, the word "an" appears before "one," "two," "three," "eleven," and "thirty" among others. This usage is consistent with the linguistic rule that "an" is used before words that begin with a vowel sound, which includes numbers that are pronounced with a vowel sound. The frequency of "an" before these numbers in the corpus further supports the claim that these numbers take the prefix "an.