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Conversation threading is a feature used by many email clients, bulletin boards, newsgroups, and Internet forums in which the software aids the user by visually grouping messages. Messages are usually grouped in a hierarchy by topic, with any replies to a message arranged visually near to the original message. A set of messages grouped in this way is called a topic thread or simply a thread. A discussion forum, e-mail client or news client is said to have "threaded topics" or a "threaded mode" if messages can be grouped in this manner.
The advantage of threaded discussions is that they allow the reader to appreciate quickly the overall structure of a conversation; specifically, who is replying to whom. As such it is most useful in situations with extended conversations or debates, such as newsgroups. Indeed, for really complex debate, it quickly becomes impossible to follow the argument without some sort of hierarchical threading system in place.
Another benefit is in the more subtle appreciation of community in hierarchically threaded systems. As responses have to be made to specific posts, they are also made to specific individuals. Threaded conversations therefore tend to focus the writer on the specific views and personality of the individual being responded to. This occurs less in forums where the latest message is just added to the end of the list.
Imposing a tree hierarchy tends to fragment discussion within a topic---messages tend to be responded to individually. It is arguable that this leads to a more confrontational debating style in forums that use threading.
When users are able to choose their personal display mode, the hierarchical structure can easily be disrupted and become unusable. Users of the flat or linear mode may append their reply to the most recent post by default, regardless of which post is actually replied to. This is a significant weakness of the linear display mode on any forum that can also be browsed in a hierarchical mode.
Unnecessary Threads 
Some forums have long-running discussions or specific threads of discussion meant to address specific topics. These often do not want new threads started unnecessarily, as large numbers of threads can become overwhelming. Additionally, by controlling the threads of discussion, these forums focus their users on specific topics and reduce the amount of off-topic messages.
Open thread 
An open thread refers to a blog post (or, in such cases as Gawker Media's blogs, hashtag) where readers may comment and discuss any topic that they choose. They are usually more useful on popular blogs with large amounts of traffic; they are often used when the author of the blog has no subject matter to post on or when there is a lull in posting.
- Both MS Outlook and Apple's Mail include the ability to organize email this way
- Dartmouth. (2003). "Taking discussion online"[dead link]
- Wolsey, T. DeVere, "Literature discussion in cyberspace: Young adolescents using threaded discussion groups to talk about books. Reading Online, 7(4), January/February 2004. Retrieved 2007-12-30.
- Network Working Group,IETF (June 2008). "Internet Message Access Protocol - SORT and THREAD Extensions". Retrieved 2009-10-10.