A vs. an

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"A" vs. "an"

Use "a" if the first letter of the following word is pronounced as a consonant.

Wrong: "A historical view of an UML diagram is better than a XML document."

Right: "An historical view of a UML diagram is better than an XML document." Note the pronunciation, which is what counts:

• "An istorical view" [NB: Americans might prefer "A historical view" after all.]
• "a yoo-em-ell diagram"
• "an ex-em-ell document"