Gnus threads articles by default. To thread is to put responses to articles directly after the articles they respond to—in a hierarchical fashion.
Threading is done by looking at the References headers of the
articles. In a perfect world, this would be enough to build pretty
trees, but unfortunately, the References header is often broken
or simply missing. Weird news propagation exacerbates the problem,
so one has to employ other heuristics to get pleasing results. A
plethora of approaches exists, as detailed in horrible detail in
Customizing Threading.
First, a quick overview of the concepts:
The top-most article in a thread; the first article in the thread.
A tree-like article structure.
A small(er) section of this tree-like structure.
Threads often lose their roots due to article expiry, or due to the root already having been read in a previous session, and not displayed in the summary buffer. We then typically have many sub-threads that really belong to one thread, but are without connecting roots. These are called loose threads.
An attempt to gather loose threads into bigger threads.
A thread where the missing articles have been “guessed” at, and are displayed as empty lines in the summary buffer.