Message lists can be "threaded" to show which messages are connected to each other and improve the readability of, for instance, related topics within folders.
You can thread messages in the message list by Reference (<Alt+1>), Subject (<Alt+2>), From (<Alt+3>) and To (<Alt+4>) - and, of course, unthreaded (<Alt+0>). View modes give you even more threading options.
Threading by Reference is the default and standard threading method. It relies on the use and presence two specific message headers: 'In-Reply-To:' and 'References:'. These message headers contain "Message ID" strings. The Bat! can thread together messages which refer back to the ID(s) of earlier messages in the topic.
The other threading methods are more simple in approach and involve chaining together messages with matching Subjects, To or From values in chronological order.
To expand all threads in a folder message list press <Shift+Ctrl+*>. To expand / collapse all branches of the current thread, use <Shift+Ctrl+Plus/Minus>. To expand and collapse threads use <Plus/Minus> and to toggle threads explanded or collapsed you can use <Shift+Space>, <Ctrl+Space> or <Alt+Space>.