If only a post's author and one comment are having a conversation then any `@` notifications between them are removed from the start of comments on the grounds they are considered noise because post owners are *always* notified of comments whether you `@` them or not.

This will not happen if any other person is involved in that comment thread.

*(For ref see http://meta.stackexchange.com/a/97290/147926 and http://meta.stackexchange.com/a/97099/147926 among probably others on MSO)*

For the sake of completeness, I guess I should point to the [**How do comment @ replies work?**][2] FAQ post, in case you want more detailed information in general.

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As a side note, comments that attempt to ping multiple users (ie, multiple `@`'s) [are also blocked][1], the work around (backticks) will cause only the first `@` to be notified.

[1]: http://meta.math.stackexchange.com/questions/2094/should-we-discourage-the-use-of-multiple-redundant-username-pings-in-comments/2615#2615
[2]: http://meta.stackexchange.com/questions/43019/how-do-comment-replies-work