diff --git a/emacs/notmuch-tree.el b/emacs/notmuch-tree.el
index 8b246a2e..68614623 100644
--- a/emacs/notmuch-tree.el
+++ b/emacs/notmuch-tree.el
@@ -97,6 +97,15 @@ different kind of arrow point."
   :type '(alist :key-type symbol :value-type string)
   :group 'notmuch-tree)
 
+(defconst notmuch-tree--field-names
+  '(choice :tag "Field"
+	   (const :tag "Date" "date")
+	   (const :tag "Authors" "authors")
+	   (const :tag "Subject" "subject")
+	   (const :tag "Tree" "tree")
+	   (const :tag "Tags" "tags")
+	   (function)))
+
 (defcustom notmuch-tree-result-format
   `(("date" . "%12s  ")
     ("authors" . "%-20s")
@@ -106,7 +115,11 @@ different kind of arrow point."
     ("tags" . "(%s)"))
   "Result formatting for tree view.
 
-Supported fields are: date, authors, subject, tree, tags.
+List of pairs of (field . format-string).  Supported field
+strings are: \"date\", \"authors\", \"subject\", \"tree\",
+\"tags\".  It is also supported to pass a function in place of a
+field-name. In this case the function is passed the thread
+object (plist) and format string.
 
 Tree means the thread tree box graphics. The field may
 also be a list in which case the formatting rules are
@@ -114,14 +127,12 @@ applied recursively and then the output of all the fields
 in the list is inserted according to format-string.
 
 Note that the author string should not contain whitespace
-\(put it in the neighbouring fields instead). For example:
-    (setq notmuch-tree-result-format
-          '((\"authors\" . \"%-40s\")
-            (\"subject\" . \"%s\")))"
-  :type '(alist :key-type (choice string
-				  (alist :key-type string
-					 :value-type string))
-		:value-type string)
+\(put it in the neighbouring fields instead)."
+
+  :type `(alist :key-type (choice ,notmuch-tree--field-names
+				  (alist :key-type ,notmuch-tree--field-names
+					 :value-type (string :tag "Format")))
+		:value-type (string :tag "Format"))
   :group 'notmuch-tree)
 
 (defcustom notmuch-unthreaded-result-format