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### 3.23 Finding the Parent

`^`
  If you'd like to read the parent of the current article, and it is not displayed in the summary buffer, you might still be able to. That is, if the current group is fetched by NNTP, the parent hasn't expired and the `References` in the current article are not mangled, you can just press `^` or `A r` (`gnus-summary-refer-parent-article`). If everything goes well, you'll get the parent. If the parent is already displayed in the summary buffer, point will just move to this article.

  If given a positive numerical prefix, fetch that many articles back into the ancestry. If given a negative numerical prefix, fetch just that ancestor. So if you say `3 ^`, Gnus will fetch the parent, the grandparent and the grandgrandparent of the current article. If you say `-3 ^`, Gnus will only fetch the grandgrandparent of the current article.

`A R (Summary)`
  Fetch all articles mentioned in the `References` header of the article (`gnus-summary-refer-references`).
`A T (Summary)`
  Display the full thread where the current article appears (`gnus-summary-refer-thread`). This command has to fetch all the headers in the current group to work, so it usually takes a while. If you do it often, you may consider setting `gnus-fetch-old-headers` to `invisible` (see Filling In Threads). This won't have any visible effects normally, but it'll make this command work a whole lot faster. Of course, it'll make group entry somewhat slow.

  The `gnus-refer-thread-limit` variable says how many old (i. e., articles before the first displayed in the current group) headers to fetch when doing this command. The default is 200. If `t`, all the available headers will be fetched. This variable can be overridden by giving the `A T` command a numerical prefix.

`M-^ (Summary)`
  You can also ask Gnus for an arbitrary article, no matter what group it belongs to. `M-^` (`gnus-summary-refer-article`) will ask you for a `Message-ID`, which is one of those long, hard-to-read thingies that look something like ‘`<38o6up$6f2@hymir.ifi.uio.no>`’. You have to get it all exactly right. No fuzzy searches, I'm afraid.

  Gnus looks for the `Message-ID` in the headers that have already been fetched, but also tries all the select methods specified by `gnus-refer-article-method` if it is not found.

If the group you are reading is located on a back end that does not support fetching by `Message-ID` very well (like `nnspool`), you can set `gnus-refer-article-method` to an NNTP method. It would, perhaps, be best if the NNTP server you consult is the one updating the spool you are reading from, but that's not really necessary.

It can also be a list of select methods, as well as the special symbol `current`, which means to use the current select method. If it is a list, Gnus will try all the methods in the list until it finds a match.

Here's an example setting that will first try the current method, and then ask Google if that fails:

         (setq gnus-refer-article-method
               '(current
                 (nnweb "google" (nnweb-type google))))

Most of the mail back ends support fetching by `Message-ID`, but do not do a particularly excellent job at it. That is, `nnmbox`, `nnbabyl`, `nnmaildir`, `nnml`, are able to locate articles from any groups, while `nnfolder`, and `nnimap` are only able to locate articles that have been posted to the current group. `nnmh` does not support this at all.

Fortunately, the special `nnregistry` back end is able to locate articles in any groups, regardless of their back end (see fetching by `Message-ID` using the registry).