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With initialisms, should one choose "a" or "an" based on the sound of the first letter of the initialism or based on what the first letter denotes?
For example, if "MOU" denotes Memorandum of Understanding, should one choose "a" or "an" based on "em" or based on "memorandum" -- a MOU or an MOU?
The choice presents an issue when the first letter of an initialism is a consonant with a vowel sound or a vowel with a consonant sound, e.g.:
- FAQ – Frequently Asked Question; a FAQ or an FAQ?
- LOC – Level of Concern; a LOC or an LOC?
- MOU – Memorandum of Understanding; a MOU or an MOU?
- NPV – Net Present Value; a NPV or an NPV?
- R&D project – Research and Development project; a R&D project or an R&D project?
- SOP – Standard Operating Procedure; a SOP or an SOP?
- XRD – X-Ray Device; a XRD or an XRD?
- URL – Uniform Resource Locator; a URL or an URL?
Also, does the choice depend on whether the writing is formal (e.g., technical, legal, policy) or informal?