A" and "an" are indefinite articles that precede nouns or even the adjectives modifying nouns. In English grammar, "a" and "an" are determiners, indicating they specify the identity or quantity of a thing, and for both phrases, that amount is "one"—the word from which they're derived. The 1st documented use https://theprettierlife.com/discovering-your-dream-home-with-heather-fabris-real-estate-your-trusted-guide-in-realizing-your-vision/