It’s an M. The consonant "M", when pronounced as an individual letter, begins with a vowel sound. Consequently, it takes the indefinite article "an".

It’s an M. The consonant "M", when pronounced as an individual letter, begins with a vowel sound. Consequently, it takes the indefinite article "an".

I bought her a necklace with an M on it.

I bought her a necklace with a M on it.

Remember, the choice between "a" and "an" is based on pronunciation, not spelling.

My favorite word, magic, begins with an m.

The chart shows an M-shaped curve.

Follow the same practice with other consonants that are spoken with a vowel sound; for example, we say an F, an H, an L, an N, an R, an S, or an X.

Name a fruit that begins with an F.

We also use "an" with other abbreviations and acronyms that begin with the letter "M" and are pronounced as individual letters, so we say an MBA program (Master of Business Administration), an MIT student (Massachusetts Institute of Technology), an MD degree (Doctor of Medicine), or an MRT line (Mass Rapid Transit).