Twitter is an American microblogging and social networking service on which users post and interact with messages known as tweets. Registered users can post, like, and retweet tweets, but unregistered users can only read those that are publicly available. Users interact with Twitter through browser or mobile frontend software, or programmatically via its APIs. Prior to April 2020 services were accessible via SMS. The service is provided by Twitter, Inc., a corporation based in San Francisco, California, and has more than 25 offices around the world. Tweets were originally restricted to 140 characters, but the limit was doubled to 280 for non-CJK languages in November 2017. Audio and video tweets remain limited to 140 seconds for most accounts.