diff --git a/doc/lispref/frames.texi b/doc/lispref/frames.texi
index 2426c9c890f..39bfed48e4e 100644
--- a/doc/lispref/frames.texi
+++ b/doc/lispref/frames.texi
@@ -420,7 +420,12 @@ Name of the physical monitor as @var{string}.
 
 @item source
 Source of the multi-monitor information as @var{string};
-e.g., @samp{XRandR 1.5}, @samp{XRandr} or @samp{Xinerama}.
+on X, it could be @samp{XRandR 1.5}, @samp{XRandr}, @samp{Xinerama},
+@samp{Gdk}, or @samp{fallback}.  The last value of @samp{source} means
+that Emacs was built without GTK and without XRandR or Xinerama
+extensions, in which case the information about multiple physical
+monitors will be provided as if they all as a whole formed a single
+monitor.
 @end table
 
 @var{x}, @var{y}, @var{width}, and @var{height} are integers.
diff --git a/lisp/frame.el b/lisp/frame.el
index 64b9f6acff8..ec3d38fa3fa 100644
--- a/lisp/frame.el
+++ b/lisp/frame.el
@@ -2465,6 +2465,10 @@ details depend on the platform and environment.
 The `source' attribute describes the source from which the
 information was obtained.  On X, this may be one of: \"Gdk\",
 \"XRandR 1.5\", \"XRandr\", \"Xinerama\", or \"fallback\".
+If it is \"fallback\", it means Emacs was built without GTK
+and without XrandR or Xinerama extensions, in which case the
+information about multiple physical monitors will be provided
+as if they all as a whole formed a single monitor.
 
 A frame is dominated by a physical monitor when either the
 largest area of the frame resides in the monitor, or the monitor