The @ symbol appears before a username to create a mention on Twitter.
A Twitter mention directly addressed another account by placing the at sign before its username, such as @Twitter. When the username was valid and the post was visible to that account, Twitter could notify the mentioned user. Mentions became a central way to reply, address people, and join conversations without sending a private message.
The symbol predates Twitter by centuries. In commercial writing, @ was used to mean “at,” including in prices and invoices. Its use in online communication grew through email and internet chat before becoming strongly associated with usernames on social networks.
The @ symbol is often confused with the hashtag symbol, but they serve different purposes. A mention targets an account, while a hashtag begins with # and helps classify or connect posts around a topic. Twitter also changed how replies and mentions appeared over time, but the basic username marker remained the at sign.