gnus-thread-sort-functions

gnus-thread-sort-functions is a variable defined in `gnus-sum.el'.
Its value is

(gnus-thread-sort-by-number)

• This variable may be risky if used as a file-local variable.

Documentation:
*List of functions used for sorting threads in the summary buffer.
By default, threads are sorted by article number.

Each function takes two threads and returns non-nil if the first
thread should be sorted before the other. If you use more than one
function, the primary sort function should be the last. You should
probably always include `gnus-thread-sort-by-number' in the list of
sorting functions -- preferably first. Also note that sorting by date
is often much slower than sorting by number, and the sorting order is
very similar. (Sorting by date means sorting by the time the message
was sent, sorting by number means sorting by arrival time.)

Each list item can also be a list `(not F)' where F is a
function; this specifies reversed sort order.

Ready-made functions include `gnus-thread-sort-by-number',
`gnus-thread-sort-by-author', `gnus-thread-sort-by-recipient'
`gnus-thread-sort-by-subject', `gnus-thread-sort-by-date',
`gnus-thread-sort-by-score', `gnus-thread-sort-by-most-recent-number',
`gnus-thread-sort-by-most-recent-date', `gnus-thread-sort-by-random',
and `gnus-thread-sort-by-total-score' (see
`gnus-thread-score-function').

When threading is turned off, the variable
`gnus-article-sort-functions' controls how articles are sorted.

By default, threads and their subthreads are sorted according to
the value of this variable. To use a different sorting order for
subthreads, customize `gnus-subthread-sort-functions'.

You can customize this variable.