A form of conferencing
in which new comments may be posted as "replies" to any posting already part of the topic. This can create a number of discussion "threads" descending from the original posting in a topic.
Usenet implements a threaded conferencing model of sorts. Slashdot, which originally had a linear conferencing system, now employs a threaded one, albeit one which can be used linearly. CommunityWare offers threaded conferencing with its "Bulletins" feature.