I believe the syntax to use is that @ symbol followed by the member's username with no spaces. That one is easy when a username is formatted as one word. What should one do if a member's username has two words?
3 Answers
As mentioned in the other answer, the most complete identifying string is with space removed, this is also what autocomplete suggests and at least in a technical sense this is the most correct string. However, anything that matches the at least first three letters (without space) will work.
It is thus quite common to use only the first part of the display name in case of spaces, especially when this feels natural given the nature of the display name. That is, many would write for example @Jules in your case, while say @The for a user named "The Mathematics Apprentice" might be less common, but could also happen. (Some care is needed in case of similar display names. If a comment of John Smith is followed by a comment of Johnny then a third with @John will notify the latter not the former, and @JohnSmith or at least @JohnS is needed to get the former).
In any case, if you write @{somestring} everything after the first space in {somestring} is completely irrelevant on a technical level. Thus "@Jules Hanson" would work just as well as "@Jules Manson" and both are technically equivalent to "@Jules"; by contrast "@JulesHanson" would not notify you. There are a few user though that still reproduce the display name with space, for reasons not exactly clear to me, but I believe they find it more aesthetic or polite or something along these lines.
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5$\begingroup$ Re: for reasons not exactly clear to me. I have even seen a user who in their profile wrote that they prefer that other users address them in comments by "name" or "name surname", i.e. with a space, $\endgroup$ Commented Dec 19, 2015 at 9:48
You should omit the space. For instance, if I wanted to reply to you in comments, I would write @JulesManson. This Meta.SE post lays out how comment replies work in general.
At the beginning of my comment, when I type @ followed by a few letters, I get suggested completions automatically. Those completions take care of what do do with spaces and special sumbols. Of course only eligible users are listed (those who have already posted), since other users will not be notified by the @ syntax anyway.
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4$\begingroup$ "Of course only eligible users are listed" notice however that not all eligible users are listed. $\endgroup$– quidCommented Dec 19, 2015 at 14:59