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A threaded discussion is an electronic discussion (such as one via e-mail, e-mail list, bulletin board, newsgroup, or Internet forum) in which the software aids the user by visually grouping messages. Messages are usually grouped visually in a hierarchy by topic. A set of messages grouped in this way is called a topic thread or simply "thread". A discussion forum, e-mail client or news client is said to have "threaded topics" if it groups messages on the same topic together for easy reading in this manner. Moreover, threaded discussions typically allow users to reply to particular posting within a topic's thread. As a result, there can be a hierarchy of discussions within the thread topic. Various types of software may allow this hierarchy to be displayed in what's called Threaded Mode. (The alternative being Linear Mode, which typically shows all of the posts in date order, regardless of who may have specifically replied to whom.)
The advantage of hierarchically threaded views 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 situation 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 fora where the latest comment is just inserted into the general pool.
A disadvantage of hierarchical threading over flat threading is an increased level of complication, and such a view therefore requires an increased level of comfort and sophistication on the part of its users. It is therefore not surprising that its takeup has been heaviest in some of the oldest and/or most sophisticated of online communities, such as Usenet, CIX or Slashdot. Web chat and comment systems are, by comparison, younger and open to a wider audience, and as such hierarchical threading is only recently becoming commonplace in such arenas.
Imposing a tree hierarchy also 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 fora that use hierarchical threading. However, true though that may be, if a direct threaded reply is no longer possible due to volume of replies to the desired post, users are now often using quotes by the person they are responding to in order to keep the conversation on track and flowing smoothly. This is recommended by most message board communities in the event that the threading has reached its otherwise comprehensive limit.
When users are able to choose their personal display mode, the hierarchical structure can easily be disrupted and become unusable: Users of the flat threading mode may append their reply to the most recent post by default, regardless of what post is actually replied to. This is a significant weakness of the hierarchical display mode on any forum that can also be browsed with a flat display.
An open thread refers to a blog post 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.
Open threads are also used to break up the monotony of posts on the main pages of blogs. Comments may build up on content-oriented posts; therefore, authors use the open threads so page load times won't be slowed down.
- Yahoo! Groups , MSN Groups  and Slashdot  all offer web-based forums that feature threaded discussions.
- Dartmouth. (2003). "Taking discussion online"
- 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.de:Thread (Internet)