Is the article before "Adjective + noun" a, or an? Whether the article in front of "Adjective + noun" uses a or an depends on the adjective or noun

Is the article before "Adjective + noun" a, or an? Whether the article in front of "Adjective + noun" uses a or an depends on the adjective or noun

Depending on the adjective, if the adjective starts with a vowel, use an, not a