Acronyms and initialisms can be presented in a number of different styles, as a result of the way they have evolved over the past century. Each has its own benefits and drawbacks and reasons for use.

With upper-case letters

As we’ve seen in the above examples, the default style in current academic writing is to use all capital letters closed up. That is, for example, ‘USA’ for ‘United States of America’. This style makes it very clear that the initials ‘USA’ are not a regular word (as regular words are written in lower-case letters), and thus most readers will assume it is an acronym or initialism. On the other hand, acronyms and initialisms written all in capitals can be confused with capitalized company names and will be obscured in titles written all in capital letters. In academic writing, almost all writers will use this style, but others are possible. As an aside, the chemical symbols for some elements were created in the same way, from the initial letter of a name of the element, such as ‘H’ for hydrogen and ‘K’ for potassium (formerly called kalium).

With periods

The earliest style, and one still sometimes used, is to place periods (i.e., dots or full stops) and even spaces between the letters. Let’s have a look at ‘United States of America’ again. With periods, we have ‘U.S.A.’ and with spaces we have ‘U. S. A.’ Some people even say ‘U. S. of A.’ Do these look familiar? This is just like the proper abbreviation I defined at the beginning: e.g., Prof., Ph.D. In this case, most of each word has been cut off, leaving only the initial letter with a period to indicate the missing letters. This is how acronyms and initialisms were first created, as full abbreviations of words. This style makes it completely clear that the letters are initials. This style was common in old documents, but is now mostly going out of use. It is still sometimes used to make it clear that a contrived acronym made to resemble a word is in fact an acronym and not a word. For example:

  • Correct but unclear: “The USA’s USA PATRIOT Act was highly controversial.”
  • Correct and clear: “The USA’s U.S.A. P.A.T.R.I.O.T. Act was highly controversial.”

Except for the regular abbreviations, it is almost never used for acronyms and initialisms in modern academic writing, as it is generally not necessary (as acronyms and initialisms should be formally defined anyway) and not efficient (using periods and spaces doubles or triples the number of characters, increasing the length and diminishing the benefits of a shortened term). I’ve never seen an academic journal style guide call for this, so you shouldn’t have to worry about it in your writing. Nevertheless, it does appear in some special mathematical notation (such as ‘without loss of generality’, or ‘w.l.o.g.’), old Latin-based notation (e.g., ‘id est’ meaning ‘it is’, which we abbreviate as ‘i.e.’ to mean ‘that is’), common shorthand phrases (like ‘with respect to’, as ‘w.r.t.’), and in the initials of names (for example, Albert Einstein as A. Einstein or A. E.), or you may come across it in old academic writing. Some of this notation even sometimes appears without the periods (such as ‘WLOG’, ‘wrt’, ‘A Einstein’ and ‘AE’). You may consider using periods to distinguish an acronym or initialism from a word that is spelled the same.

With lower-case letters

You will commonly see acronyms and initialisms that use lower-case letters or even mixtures of upper-case and lower-case letters. Let’s look at a few examples:

  • Correct: “The crystal lattice has a body-centered cubic (bcc) structure of titanium and nickel atoms.”
  • Correct: “We investigate how a hardened cement paste (hcp) is affected by a shrinkage reducing admixture (SRA).”
  • Correct: “A terminator regulates upstream genes, most likely through the influence of messenger ribonucleic acid (mRNA) stability.”
  • Correct: “A patient with chronic inflammatory demyelinating polyneuropathy (CIDP) can be treated with intravenous immunoglobulin (IVIg).”

As I discussed previously, acronyms and initialisms evolved from abbreviations that left only the initial letters of words. In this way, a word written in lower-case letters would produce a lower-case initial, while a word beginning with an upper-case letter (typically the proper name of something or someone) would produce an upper-case initial. Hence, lower-case, upper-case, and mixed-case acronyms and initialisms would be created.

In modern writing, however, all acronyms and initialisms default to upper case, regardless of the original case of the words, but some standard notation and existing acronyms and initialisms in lower case remain in use. Acronyms that have become standard words, like laser, scuba, and radar, are all written in lower case. Some acronyms and initialisms even appear in both forms: for example, for hardened cement paste, both ‘hcp’ and ‘HCP’ are in use. Furthermore, mixed cases can be used to distinguish between parts of an acronym or initialism. For example, messenger RNA (mRNA) and transfer RNA (tRNA) use lower-case ‘m’ and ‘t’ to show that the ‘messenger’ and ‘transfer’ parts of these names are not a part of the ‘ribonucleic acid’ (RNA) part of the name. I find this is common in biology and medical fields.

With non-initial letters

Many acronyms, and even some initialisms, don’t just use the initial letters of their component words, but also take second or even third letters, making them close to being portmanteaus. For example, the word ‘quantum’ and many other words starting with ‘q’ are often initialized as ‘qu’, because in English the letter ‘q’ is always followed by a ‘u’. Therefore, the quantum bit is known as a qubit (pronounced ‘cue-bit’), and the superconducting quantum interference device is known as a SQUID rather than ‘SQID’. This approach can be used to help form acronyms that one can say, rather than creating initialisms, or to distinguish a particular acronym or initialism from a word or similar acronym or initialism. In the past, this was used to create shorter names for companies and other organizations with very long names, but it is less common nowadays. However, note that the symbols for a number of chemical elements were created this way, from English or older common names: helium is denoted as ‘He’ and gold is denoted as ‘Au’ (from the Latin name aurum).

A letter can also be taken from a later part of the word, particularly if that word can be broken up into prefixes or suffixes (word parts that can be added to the beginnings or endings of words to alter their meaning). For example, in ‘quantum chromodynamics’, we have the prefix ‘chromo-’, giving us ‘quantum chromo-dynamics’, from which we can get the initialism ‘QCD’ rather than ‘QC’, emphasizing the concept of dynamics in this term. Many chemical names are made up of prefixes, giving a lot of options for initials: ribonucleic acid is RNA while deoxyribonucleic acid is DNA (but not ‘DORNA’). These letters don’t necessarily have to come from prefixes or suffixes, but from some other point in the word. This is common in chemical symbols: magnesium is Mg while manganese is Mn, as ‘Ma’ could have applied to both. However, some cases take this to extremes: FLAMINGOS is a contrived acronym for the FLoridA Multi-object Imaging Near-infrared Grism Observational Spectrometer, with letters chosen to create the name of the animal, but the ‘L’ for the second letter and ‘A’ for the last letter of ‘Florida’ are completely non-intuitive.

With other punctuation

You may also see acronyms and initialisms with hyphens (-) and slashes (/). Hyphens are typically used to stand in for spaces or to indicate a separation between parts of an acronym or initialism. For example, MS-DOS, the hybrid initialism/acronym for the Microsoft Disk Operating System uses a hyphen to indicate the distinction between ‘MS’ for the developer, Microsoft, and ‘DOS’ for the kind of computer operating system it is. Hyphens are also used to replace en dashes and the like. For example, “the Navier–Stokes (N-S) equations”. Forward slashes, meanwhile, are a convention of some two-letter initialisms used as notation or shorthand, such as ‘n/a’ (not applicable) and ‘w/o’ (without).

With other punctuation

From examples like ‘2D’ and ‘3D’ for ‘two dimensions’ and ‘three dimensions’, we’ve seen that numbers are almost always denoted by their numerals in hybrid initialisms and not by their initial letters. This is because the numeral ‘2’, for example, is a single typed character, so it saves as much space as using a letter like ‘T’, making it a very natural and efficient choice. Furthermore, using initials would produce ‘TD’ for both terms, so using the existing numerals is less ambiguous. Finally, most of the lowest numbers have only one or two syllables, and don’t require shortening in speech or in the mind’s ear when reading. Only rarely, or for large values, are numbers denoted by letters, as we saw for the FAST example above: ‘five hundred’ is initialized by ‘F’ and not ‘5’ or ‘500’.

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