It's an SMS (Short Message Service). Since the letter "S" starts with a vowel sound, the abbreviation SMS takes the indefinite article "an".

It's an SMS (Short Message Service). Since the letter "S" starts with a vowel sound, the abbreviation SMS takes the indefinite article an.

An SMS is typically short and concise.

A SMS is typically short and concise.

Remember, when choosing between "a" and "an", it's the pronunciation of the following word, and not the spelling, which is important.

You can send an SMS to an end user.

What’s the difference between a text message and an SMS message?

Similarly, we say an S, not "a S".

I've just bought a necklace with an S on it.

A partial list of abbreviations that also start with the letter "S" and take the article "a":

• an SEO (Search Engine Optimization)
• an SLA (Service-Level Agreement)
• an SMTP (Simple Mail Transfer Protocol)
• an SSD (Solid-State Drive)
• an STD (Sexually Transmitted Disease)
• an SUV (Sport Utility Vehicle)
• an SVG (Scalable Vector Graphics)