Parentheses are used to add extra information in a sentence. In academic writing, they are most often used to convey technical information such as equations, to introduce acronyms, and for in-text citations.
Sometimes you might need to use two parenthetical elements together – for example, when a sentence contains both an acronym and a citation. Style guides disagree about whether it’s okay to place two (or more) parenthetical asides side by side.
- The results were sorted by gross domestic product (GDP; Odin, 2018).
Chicago also advises this approach, but allows side-by-side parentheses if their content is entirely unrelated. If you do use two sets of parentheses, put a space between them.
When you want to enclose a set of parentheses inside another set, most style guides recommend using square brackets for the inner element.
- Several prestigious organizations (e.g., National Institutes of Health (NIH)) supported the initiative.
- Several prestigious organizations (e.g., National Institutes of Health [NIH]) supported the initiative.
In British English, parentheses within parentheses are more acceptable. However, it’s always best to avoid this kind of nesting whenever possible. If you can rephrase the sentence to remove one of the parenthetical elements, this is the best option.
- Several prestigious organizations, such as the National Institutes of Health (NIH), supported the initiative.
- This sort of testing is usually unreliable (as Jenner  took pains to show).
- This sort of testing is usually unreliable (as Jenner, 2012, took pains to show).