The rule about the usage of _a_ and _an_ as indefinite articles is that _an_ is used before a vowel sound.

- A warranty (/ˈwɑːrənti/)
- A user (/ˈjuːzər/)
- A one-way (/ˈwən ˌweɪ/)
- A man (/mæn/)
- An angel (/ˈeɪnʤəl/)
- An information (/ˌɪnfərˈmeɪʃən/)

When used before an acronym, the rule is still valid, but which article to use depends from how the acronym is pronounced (letter by letter, or as a word).

- An MP3 (/ɛm pi θri/)
- An RPG (/ɑːr pi ʤi/)
- An FBI agent (/ɛf biː aɪ/)
- A GPS device (/ʤi pi ɛs/)
- A NASA employee (/ˈnæsə/)