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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 with their replies. These groups are called a conversation, topic thread, or simply a thread. A discussion forum, e-mail client or news client is said to have a "conversation view", "threaded topics" or a "threaded mode" if messages can be grouped in this manner.
Threads can be displayed in a variety of different ways. Early messaging systems (and most modern email clients) will automatically include original message text in a reply, making each individual email into its own copy of the entire thread. Software may also arrange threads of messages within lists, such as an email inbox. These arrangements can be hierarchical, arranging messages close to their replies in a tree, or they can be linear, displaying all messages in chronological order regardless of reply relationships.
Threaded discussions allow the reader to appreciate quickly the overall structure of a conversation. As such it is most useful in situations with extended conversations or debates, such as newsgroups and complicated email chains.
Email allows messages to be targeted at particular members of the audience by using the "To" and "CC" lines. However, some message systems don't have that option. As a result, it can be difficult to determine the intended recipient of a particular message. When messages are displayed hierarchically, it's easier to visually identify the author of the previous message.
It can be difficult to absorb important information when viewing large lists of messages. Grouping threads of messages can allow users to preview messages quicker.
In group forums, allowing users to reply to threads will reduce the number of new posts shown in the list.
Some clients allow operations on entire threads of messages. For example, the text-based newsreader nn has a "kill" function which automatically deletes incoming messages based on the rules set up by the user matching the message's subject or author. This can dramatically reduce the amount of messages one has to manually check and delete.
Accurate threading of messages requires the software to identify messages that are replies to other messages.
Some algorithms used for this purpose can be unreliable. For example, email clients that use the subject line to relate messages can be fooled by two unrelated messages that happen to have the same subject line.
Modern email clients use unique identifiers in email headers to locate the parent and root message in the hierarchy. When non-compliant clients participate in discussions, they can confuse message threading as it depends on all clients respecting these optional mail standards when composing replies to messages.
Messages within a thread do not always provide the user with the same options as individual messages. For example, it may not be possible to move, star, reply to, archive, or delete individual messages that are contained within a thread.
The lack of individual message control can prevent messaging systems from being used as to-do lists (a common function of email folders). Individual messages that contain information relevant to a to-do item can easily get lost in a long thread of messages.
In messaging systems that display threads hierarchically (as opposed to linearly), discussions can easily become fragmented. Unlike systems that display messages linearly, it is much easier to reply to individual messages that are not the most recent message in the thread.
Thread fragmentation can be particularly problematic for systems that allow users to choose different display modes (hierarchical vs. linear). Users of the hierarchical display mode will reply to older messages, confusing users of the linear display mode.
The following messaging software can group and display messages by thread.
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