Early in the Usenet’s existence there was developed an innovative feature called “threads”, a method of tracking threads of discussion on a common topic by collecting together messages with the same subject no matter when they were sent. The news reader trn was one of the first programs to support message threading.
When you respond to a message by selecting “reply”, then write your reply and send it across the Internet to the server, your message is marked with the ID of the message you replied to. When your news reader downloads the messages in a newsgroup it usually displays them in threaded format, so that messages are collected together into a tree structure based on their reply-to ID’s.
Most news readers show an arrow beside a message that contains a thread, so that you can click on the arrow and the thread of messages will open. You can then follow the conversation thread by reading the messages one by one. It is considered good form to read all of the messages in a thread before you add a new message, so that you don’t post something already addressed earlier in the thread.
This thread idea is very powerful, and has influenced the design of a number of other software applications, often called “groupware” products. The most well known and widely used such program is Lotus Notes.