4chan threads are basically a generalization of the classic (HN, Reddit, etc.) threads. Classic threads are just trees of replies, where each reply has exactly

4chan threads are basically a generalization of the classic (HN, Reddit, etc.) threads. Classic threads are just trees of replies, where each reply has exactly one parent and one or more children. By contrast, 4chan threads are directed graphs of posts, in which any post can reply to any other post by its number - even itself (hard to achieve, but not impossible). It is even possible to reply to someone in the future (which sometimes happen when people reference a future "get", in which some poster will get a "nice" post number, such as 99999). There are even some frontends that render 4chan threads as graphs in the browser. I forgot their names though.