When dealing with abbreviations, should you parenthesize the abbreviation or its expansion? I have seen it both ways. Can anyone make a good case for preferring one style over the other?
The Modern Language Association (MLA) style, which is commonly used, always puts acronyms and initialisms in parentheses after the first time they are used (not the other way around). For example: The Modern Language Association (MLA) publishes its own style guide.
The MLA Handbook for Writers of Research Papers devotes an entire chapter to abbreviations.
The Associated Press Stylebook is a little less helpful in handling abbreviations. It says that the first time an item is used, it should be spelled out fully. Then, if the abbreviation or acronym is common enough to be well known publicly, it can be used on subsequent reference. AP style does not use a parenthetical explanation behind the first mention. It just uses the acronym or abbreviation after the first full mention. For example: The National Organization of Women met at the courthouse. NOW was organized in ....
The AP Style Guide also notes that certain abbreviations or acronyms are common enough to be used on first reference, that is without having been spelled out first. (As examples, CIA and FBI can be used on first reference; Central Intelligence Agency and Federal Bureau of Investigation do not need to be spelled out on first mention.) You basically have to consult the entries in the AP Stylebook for whether an abbreviation is common enough to not be spelled out.
The Economist does it like this: “European Central Bank (ECB)”, “‘quantitative easing’ (QE)”. My impression is that this is more common than the reverse pattern. It parallels the legal usage whereby a general description is given and then a shorthand—e.g., “(‘The Company’)”, “(‘The Client’)”—is introduced in parentheses and quotes.
The rule is to parenthesize the second item.
So, for example,
Bits of RNA (ribonucleic acid) mediate the conversion of genetic information into amino acids.
Bits of ribonucleic acid (RNA) mediate the conversion of genetic information into amino acids.
*Bits of (RNA) ribonucleic acid mediate the conversion of genetic information into amino acids.
*Bits of (ribonucleic acid) RNA mediate the conversion of genetic information into amino acids.
While the first style is possible without seeming incorrect (to me anyway), I think the second one is overwhelmingly more popular--it is indeed the only one discussed explicity in the other answers.
There are at least two important points to be considered while parenthesizing.
If the article covers the term several times, perhaps because it is significant to the discussion, then use the full form in the first instance along with the abbreviation in parentheses and use the abbreviation in the rest of the article. Or as recommended by the relevant style-guide, where applicable.
In situations where, say, the term is not significant or appears but only a few times in the course of the article, it is better to use the abbreviation followed by the full form in parentheses at the first reference and subsequently, use the full form. The rationale being that we should not expect the reader, while focusing on the main theme of the article, to remember the full form when he comes across it somewhere down the paragraphs.
There are other considerations as well. For some terms, the abbreviation is better known than the full form, whereas the converse may be true in case of a few others. In some cases, the same abbreviation stands for different thing in different contexts, so using the abbreviation requires circumspection and care.
Note also that the practice varies between technical literature and media publishing, as with other kinds of writing.
We have not said it all, yet.
To some extent, it depends on what one is abbreviating. In biomedical writing, the norm is to spell out the term and put the abbreviation in parentheses. If the term is long, complicated, and not normally used but the abbreviation is, and if the journal demands that all abbreviations except DNA, RNA, HIV, and AIDS be spelled out, I sometimes put the abbreviation before the spelled-out term because it's more common. If I've previously spelled out one or more of the terms in a list and am using only the abbreviation(s) for the previously spelled-out terms, then I'll be consistent and use the abbreviation for the new term and put the spelled-out version in parentheses.