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### 3.9 Threading

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:

root

  The top-most article in a thread; the first article in the thread.

thread

  A tree-like article structure.

sub-thread

  A small(er) section of this tree-like structure.

loose threads

  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.

thread gathering

  An attempt to gather loose threads into bigger threads.

sparse threads

  A thread where the missing articles have been “guessed” at, and are displayed as empty lines in the summary buffer.

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