The origin of cross posting dates back to the early days of Usenet and other similar news/bulletin systems and the way that posts are made on there (which predates IRC and IM by many years). On Usenet, It is possible to post a message to multiple newsgroups at a single time by listing multiple groups in the 'Newsgroups' header. Unless the followups are set to something else, a reply to that post will appear in all the newsgroups. While this originally was used mostly for addressing multiple groups with commonly interesting material (for example, a newsgroup about abstract games and computer AI), it is now the domain of trolls and spammers.

Fortunately, on Usenet, a cross posted message appears as hard links in the Unix file system in multiple directories, all linking to the single inode with the message itself. The advantage of this is that it takes no more space (other than the directory entry) on servers to have a message posted in one newsgroup or 5 newsgroups (or heaven forbid, 100 newsgroups).

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