Thread (online communication) (Wikipedia)

Wikipedia’s article on conversation threading.

License: CC BY-SA 4.0 — https://creativecommons.org/licenses/by-sa/4.0/

Key points

Two competing display arrangements: hierarchical/nested (a message shown close to its replies, in a tree) versus linear/flat (all messages shown in chronological order regardless of what replies to what).
A system that offers both modes at once creates a real hazard: a reply posted from the hierarchical view can attach to an old message far back in the conversation, which then surfaces out of chronological order to someone reading the same board in linear/flat mode.
Reliable threading needs an explicit parent reference for each reply; matching by subject line alone is unreliable, since two unrelated messages can coincidentally share the same subject text.