Hashtags are not case sensitive (a search for '#hashtag' will match '#HashTag' as well), but the use of embedded capitals (i.e., CamelCase) increases legibility and improves accessibility. Some characters, such as & are generally not supported as they may already serve other search functions. Other characters may be supported on a platform-by-platform basis. Most or all platforms that support hashtags permit the inclusion of letters (without diacritics), numerals, and underscores. Some platforms may require the # to be preceded with a space. Other platforms such as YouTube and Gawker Media followed in officially supporting hashtags, and real-time search aggregators such as Google Real-Time Search began supporting hashtags.ДЄ hashtag must begin with a hash (#) character followed by other characters, and is terminated by a space or end of message. During the 2010 World Cup, Twitter explicitly encouraged the use of hashtags with the temporary deployment of 'hashflags', which replaced hashtags of three-letter country codes with their respective national flags. In 2010, Twitter introduced ' Trending Topics' on the Twitter front page, displaying hashtags that are rapidly becoming popular, and the significance of trending hashtags has become so great that the company makes significant efforts to foil attempts to spam the trending list. Д«eginning July 2, 2009, Twitter began to hyperlink all hashtags in tweets to Twitter search results for the hashtagged word (and for the standard spelling of commonly misspelled words). Hashtags have since played critical roles in recent social movements such as #jesuischarlie, #BLM, and #MeToo. The hashtag gained international acceptance during the 2009вЂ“2010 Iranian election protests Twitter users used both English- and Persian-language hashtags in communications during the events. Messina's suggestion to use the hashtag was not immediately adopted by Twitter, but the convention gained popular acceptance when hashtags were used in tweets relating to the 2007 San Diego forest fires in Southern California. As in #barcamp ?ДЄ sign with a #TimeToAct hashtag at a 2014 conference How do you feel about using # (pound) for groups.

The use of the pound sign in IRC inspired Chris Messina to propose a similar system on Twitter to tag topics of interest on the microblogging network. Channels or topics that are available across an entire IRC network are prefixed with a hash symbol # (as opposed to those local to a server, which use an ampersand '&'). The pound sign was adopted for use within IRC ( Internet Relay Chat) networks around 1988 to label groups and topics. In 1970, the number sign was used to denote immediate address mode in the assembly language of the PDP-11 when placed next to a symbol or a number, and in 1978, Brian Kernighan and Dennis Ritchie used '#' in the C programming language to indicate special keywords that the C preprocessor had to process first. The number sign or hash symbol, #, has long been used in information technology to highlight specific pieces of text. Д¬hris Messina, the inventor of the hashtag

In June 2014, hashtag was added to the Oxford English Dictionary as 'a word or phrase with the symbol # in front of it, used on social media websites and apps so that you can search for all messages with the same subject'. Hashtags became entrenched in the culture of Twitter and soon emerged across Instagram, Facebook, and YouTube. Messina made no attempt to patent the use because he felt that 'they were born of the internet, and owned by no one'. The use of hashtags was first proposed by American blogger and product consultant Chris Messina in a 2007 tweet. After the initial hash symbol, a hashtag may include letters, numerals, or underscores. For example, a search within Instagram for the hashtag #bluesky returns all posts that have been tagged with that term. Hashtags are used on microblogging and photo-sharing services such as Twitter or Instagram as a form of user-generated tagging that enables cross-referencing of content by topic or theme. A hashtag is a metadata tag that is prefaced by the hash (or pound) sign, #.

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