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. An email thread is also sometimes called an email chain. Threads can be displayed in a variety of 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 or nested, arranging messages close to their replies in a tree, or they can be linear or flat, displaying all messages in chronological order regardless of reply relationships. Conversation threading as a form of interactive journalism became popular on Twitter from around 2016 onward. Authors, such as Eric Garland and Seth Abramson, began to post essays in real time, constructing them as a series of numbered tweets, each limited to 140 or 280 characters. Mechanism Internet email clients compliant with the RFC 822 standard (and its successor RFC 5322) add a unique message identifier in the Message-ID: header field of each message, e.g. Message-ID: <xNCx2XP2qgUc9Qd2uR99iHsiAaJfVoqj91ocj3tdWT@wikimedia.org> If a user creates message B by replying to message A, the mail client will add the unique message ID of message A in form of the fields In-Reply-To: <xNCx2XP2qgUc9Qd2uR99iHsiAaJfVoqj91ocj3tdWT@wikimedia.org> References: <xNCx2XP2qgUc9Qd2uR99iHsiAaJfVoqj91ocj3tdWT@wikimedia.org> to the header of reply B. RFC 5322 defines the following algorithm for populating these fields: The "In-Reply-To:" field will contain the contents of the "Message-ID:" field of the message to which this one is a reply (the "parent message"). If there is more than one parent message, then the "In-Reply-To:" field will contain the contents of all of the parents' "Message-ID:" fields. If there is no "Message-ID:" field in any of the parent messages, then the new message will have no "In- Reply-To:" field. The "References:" field will contain the contents of the parent's "References:" field (if any) followed by the contents of the parent's "Message-ID:" field (if any). If the parent message does not contain a "References:" field but does have an "In-Reply-To:"