What Is the Difference Between an Acronym and an Initialism?

What Is the Difference Between an Acronym and an Initialism?

  • Nov 30, 2023
  • 7 min read

Do you know the difference between an acronym and an initialism? Acronyms and initialisms are both types of abbreviations. Both terms refer to abbreviations made up of a set of capitalized letters (rather than a shortened word, e.g., Mrs.), but they have slightly different meanings. In this blog post, we take a closer look at the difference between acronyms and initialisms.

What Are Acronyms and Initialisms?

Acronyms and initialisms are both abbreviations that are formed by the first letter of each word in the phrase. But there are subtle differences between the two. 

Acronyms use the first letter of each word in the phrase that we pronounce as one word. For example, we often shorten President of the United States to POTUS

Initialisms also use the first letter of each word, but each letter is pronounced individually. For example, we pronounce R-S-P-C-A for the acronym for the Royal Society for the Prevention of Cruelty to Animals

The table below gives some examples of both.

AcronymsInitialisms
NATO (North Atlantic Treaty Organization)BBC (British Broadcasting Corporation)
NASA (National Aeronautics and Space Administration)FBI (Federal Bureau of Investigation)
UNESCO (United Nations Educational, Scientific and Cultural Organization)IT (Information Technology)
GIF (Graphics Interchange Format)PDF (Portable Document Format)
PIN (Personal Identification Number)BLT  (Portable Document Format)

How to Introduce Acronyms and Initialisms

You should always introduce an abbreviation, whether an acronym or initialism, in full the first time you use it, unless it is ubiquitous or certain to be familiar to the reader. For example, few people are likely to know what MICR stands for, so the first time you use it, you should write it in full, with the acronym or initialism in brackets: 

Magnetic-ink character recognition (MICR) 

After introducing the term, you can just use the acronym or initialism. 

However, it’s likely that most people will know what TV stands for, so you don’t have to spell it out, unless the style guide you are using specifies that all acronyms and initialisms must be spelled out. 

As a proofreader, you can either fix this in the text (if you know for sure what term the client has abbreviated) or leave a note for them to introduce the term in full, with the abbreviation in brackets.

Do Acronyms and Initialisms Need Articles?

As a general rule, when an acronym is a proper noun, no articles (i.e., the, a/an) are needed:

She works for NASA.

But this is, however, only true when the acronym is used as a noun. When you use an acronym to qualify another word or term, articles are used where appropriate:

She is a NASA employee.

The NASA mission was a success.

In the first example, NASA is qualifying employee. Without the acronym, we would not know where this person works, just that she is an employee. 

In the second example, NASA is qualifying mission. But, in this sentence, we are talking about a specific mission that NASA conducted, hence the use of the definite article the

If an acronym is not a proper noun, you will generally add an article in front of it:

He reacted with a GIF of a keyboard-playing cat.

I can’t use my card because I’ve forgotten the PIN.


Initialisms work a little differently. An initialism that is a proper noun will typically require a definite article:

She works for the FBI.


But you should not use articles with initialisms that refer to substances, diseases, or conditions:

DDT is banned in some countries.

He suffers from RSI.


Again, though, this is only the case where you are using an initialism as a noun. Where you are using an initialism to qualify another word or term, use an article:

The DDT ban exists to counter its adverse effects on the environment.

Here, DDT is modifying ban to specify the type of ban the person is referring to. 

Which Article Should I Use?


In English, we typically use a before words beginning with a consonant sound and an before words beginning with a vowel sound. This is also true of acronyms and initialisms. However, the names of some consonants (e.g., F, M) begin with vowel sounds, while the vowel U starts with a consonant sound. It’s important to remember here that it is the sound rather than the letter that determines which indefinite article you should use:

An FBI agent saw a UFO.

A GIF of an ABBA album cover.

How to Punctuate Acronyms and Initialisms

As a rule of thumb, acronyms and initialisms do not have periods between the letters in UK English. Over time, this style of punctuation (e.g., C.E.O.) has also largely fallen out of vogue in US English, where it used to be much more common. The major exception to this is with the terms US and USA.

In US English, the general rule is to use periods when you use U.S. or U.S.A. as a noun: 

U.S. and U.S.A. 

I am visiting the U.S. next week. 

But, when you use US or USA as an adjective or a modifier, periods are unnecessary: 

US and USA

The US company, Amazon, is one of the highest grossing companies in history. 

Of course, this very much depends on the style guide you are using. If you are uncertain about whether a particular acronym or initialism should have periods between the letters, it is always advisable to check the client’s style guide and adapt the style accordingly.

How to Capitalize Acronyms and Initialisms

Use all caps to write most acronyms and initialisms. There are, however, a few exceptions.

You almost always capitalize initialisms, regardless of dialect. US English treats acronyms the same way. However, some British and Australian English style guides suggest only capitalizing the initial letter of acronyms.

A few specific acronyms and initialisms are lowercase except where they appear at the beginning of a sentence (in which case we capitalize the first letter). Most common among these are e.g. and i.e.

Additionally, some acronyms have become so well known that we consider them words. For many of these words, it’s no longer common knowledge that they were acronyms to begin with. 

The table below gives some examples.

LaserLight Amplification by Stimulated Emission of Radiation
RadarRadio Detection and Ranging
ScubaSelf-Contained Underwater Breathing Apparatus
TaserThomas A. Smith Electric Rifle

Capitalization can also be a matter of stylistic preference. For example, while PIN is common, the Guardian recommends pin number and the BBC recommends Pin number. Although number is redundant here, it is commonly used, so many news sites still include it in their style guides.

As there is room for variation here, it’s always best to check your client’s style guide.

How to Make Acronyms and Initialisms Plural

You can pluralize acronyms and initialisms in the same way as any other noun, but failing to do so is a common error: 

Portable Document Format > PDF 

Then:

Portable Document Formats > PDF ❌

You should treat acronyms and initialisms the same way as other nouns when making them plural, so:

Portable Document Formats > PDFs  ✅

Note that you should not capitalize the s, as it does not represent a word.

Proofreading Acronyms and Initialisms

Your main goal as a proofreader should always be to ensure clarity and consistency. As such, there may be exceptions to the rules set out in this post. If you are unsure about anything, your first port of call should always be to check the client’s style guide. If no guidance is available, concentrate on clarity and consistency, and leave a comment for the client if necessary.

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Join the Conversation

Vladimir says:
January 31, 2023 at 2:02PM
Please, for the love of everything you hold dear, tell me that you chose the RAS Syndrome example of 'PIN number' on purpose, just to mess with people!
    Knowadays says:
    February 2, 2023 at 3:41PM
    Hi Vladimir. PIN number is a great example of how capitalization can be a matter of stylistic preference! Although 'number' is redundant, it is in common usage. Many news sites, for example, still include it in their style guides. We've updated that section of the blog to make it a little clearer.

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