("Pound sign" in the UK means "£"; "#" is called hash, gate, and occasionally octothorpe.) In 1970, for example, the number sign was used to denote immediate address mode in the assembly language of the PDP-11 In the 1986 SGML standard, ISO 8886 (q.v.), # is a reserved name indicator (rni) which precedes keyword syntactic literals, --e.g., the primitive content token #PCDATA, used for parsed character data.
The pound sign was adopted for use within IRC networks circa 1988 to label groups and topics.
Three hashtags are seen by some as the "absolute maximum", and any contribution exceeding this risks "raising the ire of the community." As well as frustrating other users, the misuse of hashtags can lead to account suspensions.
Twitter warns that adding hashtags to unrelated tweets, or repeated use of the same hashtag without adding to a conversation, could cause an account to be filtered from search, or suspended.
In order for topics to trend, there has to be a consensus, whether silent or stated, that the hashtag refers to that specific topic.
Hashtags also function as beacons in order for users to find and "follow" (subscribe) or "list" (organize into public contact lists) other users of similar interest.
A hashtag is a type of metadata tag used on social networks such as Twitter and other microblogging services, allowing users to apply dynamic, user-generated tagging that makes it possible for others to easily find messages with a specific theme or content; it allows easy, informal markup of folk taxonomy without need of any formal taxonomy or markup language.
It is always safe to precede the “#” with a space, and to include letters without diacritics, digits, and underscores.The hashtag may contain letters, digits, and underscores.For example, on the photo-sharing service Instagram, the hashtag #bluesky allows users to find all the posts that have been tagged using that hashtag.Hashtags are mostly used in unmoderated, ad hoc discussion forums; any combination of characters led by a hash symbol is a hashtag, and any hashtag, if promoted by enough individuals, can "trend" and attract more individual users to discussion.On Twitter, when a hashtag becomes extremely popular, it will appear in the "Trending Topics" area of a user's homepage.Twitter has an algorithm to tackle attempts to spam the trending list and ensure that hashtags trend naturally.Although the hashtag started out most popularly on Twitter as the main social media platform for this use, the use has extended to other social media sites including Instagram, Facebook, Flickr, Tumblr, and Google .In many cases other characters are also allowed, in particular accented characters used in many languages, but handling may vary from one client to another, and from time to time as standards evolve.A discussion of hashtag standards suggests that if #Romeo&Juliet is used, different Twitter clients might link to #Romeo, #Romeo&, or #Romeo&Juliet.Hashtags intended for discussion of a particular event tend to use an obscure wording to avoid being caught up with generic conversations on similar subjects, such as a cake festival using #cakefestival rather than simply #cake.However, this can also make it difficult for topics to become "trending topics" because people often use different spelling or words to refer to the same topic.