At-Mentions / @ Mentions in Tweets
Let's say I make a tweet from my WineIntro account that says
I had a great Kung Fu Girl riesling this afternoon with my salad. What are you drinking? #WineWednesday
Let's say WineLover reads that and uses the "reply" button to respond:
@WineIntro That riesling is awesome! I'm drinking some Velvet Devil merlot.
WineLover is replying to my post, using an @-mention of my name. I get a special notice of that reply tweet because I am directly mentioned in it. That's good, because it helps me to manage people asking me questions or talking to me. It's important to note that, because WineLover used an @ name as the VERY FIRST WORD in a reply post, that the ONLY people who will see that post are him, me, and people who happen to follow BOTH of us. That privacy level is because of the special combination of the @-name use as the first word in the reply to my tweet.
So if I then hit reply to his tweet and respond with:
@WineLover Velvet Devil merlot is delicious! I'll have some later with some black forest cake.
I have also now done a reply tweet involving a leading @-name mention. My message is only seen by him, me, and any fans who follow us both.
In comparison, if the @-mention is anywhere but the very first position, the message will be seen by everyone. Some people put a period as the very first character for this reason.
If I send the reply
@WineLover Bring me some merlot over later!
That is private between me, WineLover, and our mutual friends.
If I instead send the reply
.@WineLover Let's celebrate together
or if I switch it around to read
Let's celebrate together, @WineLover
Both of those are seen by the world. The name is no longer the very first part of the tweet.
Here's an example to show just how complicated this can get.
I made an initial post from LisaSheaAuthor. My post read:

Another author, Timothy S. Bennett, retweeted my post. He added a comment to it.

I replied to his post. Note that this is the first reply in the chain. His action was to retweet my post with a comment. So in my reply I was careful to move his name out of the primary spot. That way all of my followers could see the reply.
He then replied to me, and in his reply he kept my name in the primary spot. That meant that his reply could only be seen by me, by him, and by people who followed us both.

Most of the time, of course, it's not that confusing :).
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