The easy rule, right 80% of the time, is that "a" is the indefinite article used before a noun starting with a consonant, and "an" is the indefinite article for use before a noun starting with a vowel.

But to be 100% correct is more challenging. The reason for invoking "an" is to avoid the awkward tongue twisting associated with saying "a eel". It's much easier to say "an eel." Here English shares something with French, in that the correct grammatical form depends on what sounds right or is easy to say. So the correct, more challenging rule is that "an" appears before nouns that start with vowel sounds, in contrast to written vowels. My favorite examples from my field: "a scanning probe microscope", but "an SPM" (pronounced ES-PEE-EM); "an ultra-high vacuum chamber", but "a UHV chamber" (pronounced YEW-AICH-VEE).

Place "a" or "an" in the blanks.

1. Please send me an email.
2. The system is a eutectic (YEW-TEC-TIC) at 300K.
3. The temperature was increased by an electron beam heater.
4. We used a lateral force microscope (LFM).
5. We used an LFM. (EL-EF-EM)
6. An 8 (EIGHT) ML overlayer of Pd was deposited on the sample.
7. A uniform (YEW-NI-FORM) layer was achieved.
8. An oxygen dosing was the next step in the preparation.
9. The system can be modeled by a harmonic oscillator.
10. The system can be modeled by an anharmonic oscillator.
11. An Auger electron spectrometer (AES) was employed....
12. An AES was employed....
13. An X-ray (EX-RAY) photoelectron spectrometer (XPS) revealed...
14. An XPS (EX-PEE-ES) revealed...
15. Use of a scanning electron microscope (SEM) showed...
16. Use of an SEM showed....
17. We used a fourier-transform infrared spectrometer (FTIR) to determine...
18. We used an FTIR (EF-TEE-EYE-ARE) to determine...
19. An ultra-violet photoelectron spectrometer (UPS) displayed....
20. A UPS (YEW-PEE-ES) displayed...

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Created May 7, 1998, by Nancy Burnham and Fred Hutson.