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 them. Users access Twitter through its website interface or its mobile-device application software ("app"), though the service could also be accessed via SMS before April 2020. 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.