Abbreviations

Q. Dear Sir or Madam: My client prefers to use the article a before an abbreviation such as LCMOS. I suggest using an, since the letter L is pronounced “ell.” What does the Chicago Manual of Style recommend?

A. When an abbreviation follows an indefinite article, the choice of a or an is determined by the way the abbreviation would be read aloud, so in this case, we would use an. Please see CMOS 10.9 for more information and examples.

[This answer relies on the 17th edition of CMOS (2017) unless otherwise noted.]