When the service first launched, Twitter users were restricted to posts containing one line of up of to 140 consecutive characters. While the character count hasn't changed, support for line breaks was added in March 2013, giving Tweeters the option to expand thoughts -- down to individual letters -- across several lines.
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Adding a Line Break
Line breaks give you some creativity with your posts, making your tweet more readable and your message stand out a bit. To implement the feature, simply press the "Return" button while composing a new post to add the extra line to your message.
While this method of adding line breaks applies to those posts composed using the Twitter Web interface or Twitter's official mobile apps, a handful of other applications offered line break support even before the March 2013 update. The method of inserting them, however, may vary slightly.