What Is the Difference Between an Acronym and an Initialism?

Acronyms and initialisms are both types of abbreviations. For most people, there’s no pressing need to know the difference between the two. However, most people aren’t professional proofreaders. For this rare breed, understanding the difference between an acronym and an initialism is necessary.

In this blog post, we look closely at acronyms and initialisms, focusing on the difference between the two.

What Are Acronyms and Initialisms?

We all know that an abbreviation is a term’s shortened form. But there are many types of abbreviations, two of which are acronyms and initialisms. Both terms refer to abbreviations made up of a set of capitalized letters (rather than a shortened word, e.g., Mrs.)

The major difference between acronyms and initialisms is that an acronym can be (and is) pronounced as a single word, while an initialism is pronounced as a series of letters. The table below gives some examples of both.


Rarely, an abbreviation may have elements of both acronyms and initialisms (e.g., JPEG), but this post focuses on acronyms and initialisms as distinct entities.

Introducing Acronyms and Initialisms

Whether an acronym or an initialism, you should always introduce an abbreviation in full the first time it is used unless it is ubiquitous or certain to be familiar to the reader. For instance, this means that magnetic-ink character recognition (MICR) should be spelled out in full as it is here, whereas TV for television is so common as to not require spelling out.

As a proofreader, you can either fix this in the text (if you know for sure what term is being abbreviated) or leave a note for the client 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.

This is, however, only true when the acronym is used as a noun. When it is used to qualify another word or term, articles are used where appropriate:

She is a NASA employee.

The NASA mission was a success.

If an acronym is not a proper noun, meanwhile, it will generally take an article:

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 initialisms that refer to substances, diseases, or conditions, for example, are usually used without articles:

DDT is banned in some countries.

He suffers from RSI.

Again, though, this is only the case where it is used as a noun. Where an initialism is used to qualify another word or term, articles are used:

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

In English, a is typically used before words beginning with a consonant sound, while an is used 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 should be used:

An FBI agent saw a UFO.

A GIF of an ABBA album cover.


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, which are frequently styled as U.S. and U.S.A. in US English.

Of course, this very much depends on the style guide being used. 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.


The overwhelming majority of acronyms and initialisms are written in all caps. There are, however, a few exceptions.

Initialisms are almost always capitalized in their entirety, regardless of dialect. American English treats acronyms the same way. However, some British and Australian English style guides suggest only capitalizing the initial letter of acronyms.

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

Additionally, some acronyms have become so well known that they are considered words in their own right. For many of these words, it is no longer common knowledge that they were acronyms to begin with. Some such words include:

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.


Acronyms and initialisms can be made plural like any other word, but failing to do so is a common error. The logic is sound: if Portable Digital Format is abbreviated to PDF, then it makes sense that Portable Digital Formats should also be abbreviated to PDF. However, in practice, this would be confusing.

Acronyms and initialisms should be treated like normal words when making them plural. The plural of PDF, then, is PDFs. Note that the s is not capitalized, as it does not represent a word.

Ensuring Clarity and Exceptions

As a proofreader, your main goal 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 given, 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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