a and an

Use "a" before any acronym or word that begins with a consonant sound. Use "an" before any acronym or word that begins with a vowel sound. An acronym is pronounced as a word (for example, a HEPA filter); an initialism is pronounced as its letters (for example, an NGO). The first sound of the word or letters indicates whether to use "a" or "an." Examples: a light-water reactor, an LWR; a Human Resources Office memo, an HRO memo; a nongovernmental organization, an NGO; a National Renewable Energy Laboratory subcontract, an NREL subcontract.