'A' and 'An': Which Gets Used Where | Merriam-Webster
https://www.merriam-webster.com/grammar/a-and-an-and-the-patterns-of-their-use
WEBThe patterns that determine which article, a or an, is conventionally used before a given word are based on phonetics, but the patterns exist in writing as well as speech. When preceding a consonant sound, a is used: "a cake," "a slice of cake."
DA: 24 PA: 67 MOZ Rank: 56