A" and "an" are indefinite content that precede nouns or the adjectives modifying nouns. In English grammar, "a" and "an" are determiners, this means they specify the identity or amount of a thing, and for both of those words, that quantity is "a single"—the word from which They are derived. https://theprettierlife.com/discovering-your-dream-home-with-heather-fabris-real-estate-your-trusted-guide-in-realizing-your-vision/