To reply to someone's Tweet, simply put the "@" symbol in front of their username. This makes the tweet appear to them in the reply column of their home page. This is a very useful way of starting conversations with people in order to build relationships (even people who do not follow you will see your @replies, unless they have blocked you). Start a conversation by replying to tweets that you find interesting.
An @reply ("at reply") is a Twitter update that begins with @username. To get the most out of Twitter, participate in conversations. You can easily reply to another person's tweet using an "at reply". To send someone a reply, start your tweet with @username so that it will appear in the recipient's replies tab.
A mention is any update where the text of the tweet contains @username. This is very similar to a reply. A mention is a good way to send information to multiple people (your tweet can contain multiple @usernames). You can also use it to attribute information to the original person. Even people who are not following you will see it when you mention them, when they click their mentions tab.
All tweets containing @username are saved for you, you can get to them by clicking a link in your sidebar. Click on your username to see the replies other people made to you, and mentions of your username. In order to appear in the recipients replies tab, replies and mentions must use the username. Replies and mentions from people you've blocked will not appear.
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