When the help center doesn't have the information you seek, check the main Stack Exchange meta site, starting with questions tagged FAQ. There is an FAQ on comment @replies.

The main rules are:

• @-replies only work in comments. If you type @ in a post, it's just an ordinary character, there's no way to notify a user via a post's content. (Also in chat with different rules that I won't discuss here.)
• You can only @-reply to a previous comment in the same post, to the poster, or to an editor of the post (plus a few more edge cases, see the FAQ for the full story).
• You can only have at most one @-reply in a comment.
• The author of the post you commented on is always notified, in addition to the user you @-addressed.
• Sometimes your @-mention will be erased when you post the comment. Don't worry, that only happens if that user would have been notified anyway without the @-mention.
• To notify a user, type @ and the first letter of their username, and let completion happen. If you want to type the name, it's the full name without spaces, or a prefix of at least 3 letters. See the FAQ for the nitty-gritty on abbreviations when some users have similar names.

When the help center doesn't have the information you seek, check the main Stack Exchange meta site, starting with questions tagged FAQ. There is an FAQ on comment @replies.

The main rules are:

• @-replies only work in comments. If you type @ in a post, it's just an ordinary character, there's no way to notify a user via a post's content. (Also in chat with different rules that I won't discuss here.)
• You can only @-reply to a previous comment in the same post, to the poster, or to an editor of the post (plus a few more edge cases, see the FAQ for the full story).
• You can only have at most one @-reply in a comment.
• The author of the post you commented on is always notified, in addition to the user you @-addressed.
• Sometimes your @-mention will be erased when you post the comment. Don't worry, that only happens if that user would have been notified anyway without the @-mention.
• To notify a user, type @ and the first letter of their username, and let completion happen. If you want to type the name, it's the full name without spaces, or a prefix of at least 3 letters. See the FAQ for the nitty-gritty on abbreviations when some users have similar names.

When the help center doesn't have the information you seek, check the main Stack Exchange meta site, starting with questions tagged FAQ. There is an FAQ on comment @replies.

The main rules are:

• @-replies only work in comments. If you type @ in a post, it's just an ordinary character, there's no way to notify a user via a post's content. (Also in chat with different rules that I won't discuss here.)
• You can only @-reply to a previous comment in the same post, to the poster, or to an editor of the post (plus a few more edge cases, see the FAQ for the full story).
• You can only have at most one @-reply in a comment.
• The author of the post you commented on is always notified, in addition to the user you @-addressed.
• Sometimes your @-mention will be erased when you post the comment. Don't worry, that only happens if that user would have been notified anyway without the @-mention.
• To notify a user, type @ and the first letter of their username, and let completion happen. If you want to type the name, it's the full name without spaces, or a prefix of at least 3 letters. See the FAQ for the nitty-gritty on abbreviations when some users have similar names.

When the help center doesn't have the information you seek, check the main Stack Exchange meta site, starting with questions tagged FAQ. There is an FAQ on comment @replies.

The main rules are:

• @-replies only work in comments. If you type @ in a post, it's just an ordinary character, there's no way to notify a user via a post's content. (Also in chat with different rules that I won't discuss here.)
• You can only @-reply to a previous comment in the same post, to the poster, or to an editor of the post (plus a few more edge cases, see the FAQ for the full story).
• You can only have at most one @-reply in a comment.
• The author of the post you commented on is always notified, in addition to the user you @-addressed.
• Sometimes your @-mention will be erased when you post the comment. Don't worry, that only happens if that user would have been notified anyway without the @-mention.
• To notify a user, type @ and the first letter of their username, and let completion happen. If you want to type the name, it's the full name without spaces, or a prefix of at least 3 letters. See the FAQ for the nitty-gritty on abbreviations when some users have similar names.