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.
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.
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.
A hashtag must begin with a hash character followed by other characters, and is terminated by a space, or end of message.
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.
They cannot be "retired" from public usage, meaning that any given hashtag can theoretically be used in perpetuity.
They do not contain any set definitions, meaning that a single hashtag can be used for any number of purposes, as chosen by the creators of them.
Twitter uses a different syntax for Chinese characters and orthographies with similar spacing conventions: the hashtag contains unspaced characters, separated from preceding and following text by spaces (e.g.
'我 #爱 你' instead of '我#爱你') or by zero-width non-joiner characters before and after the hashtagged element, to retain a linguistically natural appearance (displaying as unspaced '我#爱你', but with invisible non-joiners delimiting the hashtag).