The general style guide is to "write it how you say it": https://en.wikipedia.org/wiki/Article_(grammar)#Indefinite_article https://en.wikipedia.org/wiki/Artic

The general style guide is to "write it how you say it": https://en.wikipedia.org/wiki/Article_(grammar)#Indefinite_article https://en.wikipedia.org/wiki/Article_(grammar)#Indefinite_a... https://www.chicagomanualofstyle.org/qanda/data/faq/topics/Abbreviations/faq0005.html https://www.chicagomanualofstyle.org/qanda/data/faq/topics/A... Of course there will be variants where you'd just have to make a decision (e.g. how do you pronounce SQL?) One note: technically, 'HN' is an initialism; acronyms are special cases of initialisms that are pronounceable.