Would you say an NHL referee or a NHL referee? It's an NHL referee. The indefinite article an is used before all vowel sounds except the long u sound (a united stand) and before the silent h (an honourable politician). And since the first letter of NHL makes the vowel sound en, it takes the article an.
Here are some questions to test your knowledge.
Note: Adapted from Charles Skeete's article "A Question of Sound, Not Sight" in Terminology Update, Vol. 29, No. 3, 1996, p. 14.
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