Good question, and one I’ve been asking myself a lot while I’ve been writing lately.
I like to use parentheses (usually to provide a nice rhythm which would otherwise run on), and often find myself using them at the end of a sentence (like here for example).
It also seemed obvious enough to me that in most cases (as in the sentence above), the full stop (or period for those of you of a Columbian persuasion) would go outside the parentheses, as the full stop is putting to a stop to the whole sentence, which contains the parentheses (whereas putting the full stop within the parentheses would only apply to the words inside there with it).
I realised though, that I wasn’t basing this on any external authority, and began to want to make sure that I was right, and to see if there were perhaps any cases which would call for full stops within parentheses.
Fortunately enough there’s a pretty simple rule to follow (and I think I’ve generally been following it).
Basically, if the words within the parentheses don’t form a complete sentence, or are part of the sentence in which the parentheses begin, then the full stop goes outside (for the reasons I mentioned above).
But if the words in the parentheses form a complete sentence (or sentences), then the full stop goes inside, and the parentheses should come after the full stop of the previous sentence.
I like cooking (and I’m pretty good at it).
I like cooking. (I also think I’m pretty good at it.)
I’m conveying the same information in both cases, but in the first, the phrase in the brackets is connected grammatically to the first three words, and both could go together as a sentence simply by removing the parentheses. The meaning would be slightly different as I’d be foregrounding my estimation of my cooking more than if I set it aside a little by putting it in brackets. But still, it’d work.
In the second example, I’ve split the information across two sentences, but I still want to set aside my estimation of my cooking, or de-emphasise it a little (because I’m so modest), so I put it in brackets.
Generally, you’ll probably find yourself only putting parts of a sentence or sentence fragments in parentheses, so don’t worry too much about where to put your full stop: it’ll usually be on the outside.
As always though, there are grey areas and exceptions, and I’m sure I’ve not always followed the rules because of this. (Have a look through my other posts, or even this one: I’m sure I’ve broken the rule a few times.) A lot of people, for example, simply state that the rule is as follows: part of a sentence = outside brackets, complete sentence = inside brackets.
You might have noticed though, that I also mentioned sentence fragments as well as parts of sentences. Because sometimes, you might want to put something in brackets that doesn’t make a complete sentence, but also isn’t just a part of the sentence that you want to separate a little from the rest of it.
That would kind of fall between the two stools/rules, but for me for it definitely belongs in the full-stop-outside-brackets category. Putting a fragment after a full stop, and inside parentheses alongside its own full stop, would be treating it too much like a full sentence, which just feels wrong when reading it.
And then there are times when you might want to include a full sentence (I love full sentences) within parentheses, within a longer sentence (like this one). Luckily, that is generally accepted and most guides suggest it’s fine, but not to capitalise the sentence (unless the example you provide begins with a word like I… ahem).
Oh, and what if the information you include in parentheses affects the grammar of the sentence as a whole!? (!!) For example:
The president (and his entourage) are expected to arrive in the morning.
Some purists will tell you that this is wrong, because removing the parentheses makes the sentence grammatically incorrect (The president are expected…). But I think it’s fine, as reading it or saying it loud sound absolutely fine (and really, who’s going to go the effort of reading the sentence without the parentheses and getting their knickers in a twist?)
Hang on, what about that last sentence, which began as a statement, but then had parenthetical information added in the form of a question? Should the question mark be inside or outside the parentheses, or should there even be a question mark at all!? Honestly, what kind of loose cannon just goes around sticking questions to the ends of sentences like that?? (I mean really!!)
You can see how this could get really complicated (and why I’m sure I’ve broken the rules many times). So my advice is not to overthink it, and if what you write sounds OK when you read it back to yourself, it’s fine (unless perhaps you’re writing for a publication with a strict style guide, or in academic writing, when you know your assessor is a stickler for the rules). But if you want to break the rules for stylistic effect? Go for it (and if they complain, tell them I sent you).