Readers often ask a grammar question about whether to use a or an. This often occurs because they see a before a word beginning with a vowel, and they have been taught to use an before a vowel.
The short answer is that it depends on the sound of the noun or adjective following the determiner.
Let’s begin by identifying a or an as one of the 8 parts of speech. A and an are articles, like the. Articles are classified as determiners, which are a type of adjective. Whether we use a/an or the depends on whether we are referring to a specific example of a noun, or the general concept. (i.e. the blue car that is parked across from my house or a blue car, which could be any one of millions of blue cars). A determiner specifies a noun.
When the noun begins with a consonant sound, then we can use a because it’s easy to pronounce (i.e. a blue car). But English has evolved a second form for words that begin with vowel sounds (i.e. an aardvark). But some vowel sounds can be hard, they go better with a than an. One example is the long u sound. Therefore we say a university, not an university. Similarly, we say a European, not an European.
On the other hand, sometimes the initial consonant in a word is silent, as in honor or herb. So with these words, we use an.
A or an with acronyms or abbreviations
Since it’s the sound of the letter that guides us, when the article precedes an acronym or abbreviation, we have to consider how it will sound when read. An acronym is a word like Scuba (which means Self Contained Underwater Breathing Apparatus). It’s made of the first letters of each of the words in the phrase. Sometimes acronyms skip a letter or use a vowel from one of the words to make them readable. Other abbreviations such as SPCA (Society for the Prevention of Cruelty to Animals) are spoken with each letter sounded out (ess-pee-cee-eh). For such words, use the beginning sound of the word as spoken. For example: “I found a dog; I need to find an SPCA shelter for it.”
A historic or an historic?
One challenging question is a historic or an historic. Oxford Dictionaries supports the use of an here as does Grammar Girl. A Google Ngram plot of the two usages shows the two usages as similar (with “an historic” leading) until the 20th century. From the mid-20th century, the usage of an historic has been falling, and now is clearly not preferred.
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